Russian uses different grammatical cases to express various aspects of time.
Each case provides a unique perspective, whether it's a specific moment, an extended period, a background atmosphere, or a precise calendar date.
For short units like second, minute, hour, and day, we use the Accusative case. We also use it when talking about historical or symbolic periods, eras.
For longer units like week, month, year, or century, we usually use the Prepositional case.
Time of the day or seasons are in Instrumental.
And specific dates are in the Genitive case - precise calendar points.
Let's go into detail.

The Accusative case expresses a specific point in time or a limited period - small time units like a moment, an hour, a day, or a specific occasion. Think of it like a single dot or placing a pin on the timeline. This case captures the exact moment when an event occurs.

• В тот момент (at that moment)
• В это мгновение (at this instant)
• В этот раз, в это время (at this time)
• В эту секунду (at this second)
• В три часа (at three o clock)
• В пять часов одну минуту (at five hours and one minute / at 5:01)

But:

• В скором времени (in the near future, soon) - here we use the Prepositional case, not Accusative.

We use В + Accusative with days and days of the week:

• В этот день (on this day)
• В прошлый понедельник (last Monday)
• В эту пятницу (this Friday)
• В следующие выходные (next weekend)

But:

• На следующий день (the next day) - with На + Accusative
• На выходных (on the weekend / during the weekend) Prepositional case

We use Accusative with holidays:

• На Новый год - В Новый год (at New Year)
• В/ на День независимости (on Independence Day)
• На Пасху (at Easter)

Accusative is also used for historical or symbolic periods:

• В эпоху застоя (during the era of stagnation)
• В наше время (in our time)
• В период войны (during the war period)
• В мезозойскую эру in the Mesozoic era

Special cases and exceptions:
Let's see the difference between в выходные and на выходных - both are correct.

1. в выходные - on the weekend / during the weekend. This is Accusative, used when we describe an event happening in that period.

• Мы поедем в деревню в выходные (We ll go to the countryside on the weekend.)
• В выходные обычно отдыхаю (I usually rest on weekends.)

Focus: the time period as a point on the calendar (when? - в выходные).

2. на выходных - on the weekend / over the weekend. This is Prepositional case, used idiomatically in modern Russian.

• Мы встретились на выходных (We met over the weekend.)
• Что ты делал на выходных? (What did you do on the weekend?)

But in practice: both в выходные and на выходных are correct. I'd say that в выходные more neutral/literary or formal, often used in written Russian or in news.
And на выходных is more colloquial, natural in conversation.

Another special case is when we use в or на with holidays.
Праздники. В День независимости - На День независимости - literally on Independence Day.
1. We use В День независимости when we mean the time when something happens - the day itself as a point on the timeline.

• В День независимости проходят парады(Parades take place on Independence Day.)
• Мы познакомились в День независимости (We met on Independence Day.)

Focus: the moment/day when something happens. It answers "Когда"

• Я загадала желание в Новый год - I made a wish on New Year s Eve. - at that night.

2. When are we saying на?

• На День независимость - literally for (the occasion of) Independence Day. We use На + Accusative when we mean the purpose or the occasion, it's connected with the holiday.
• Мы поедем в столицу на День независимости (We ll go to the capital for Independence Day / to celebrate it.)
• Она купила платье на День независимости (She bought a dress for Independence Day.) - for the occasion

Focus: the event or goal (celebration, plans, reason).It would answer more not "Когда?" but "К чему? Для чего?" (for what occasion?)

And the same for На Новый год

• На Новый год мы поехали в горы. For New Year s, we went to the mountains (during the holiday period).
• Мне подарили книгу на Новый год - They gave me a book for New Years

They're almost interchangeable, but в focuses on the day itself (when), while на highlights the occasion or celebration (for what).

The Prepositional case is used for a period, something that lasts. It's used with weeks, months, years, centuries.
You can imagine it as a container that holds time. You open the box and see everything that happened inside that period - it's not a single moment, but rather a space where events unfold.

Examples:

• Неделя. На + Prepositional case: На следующей неделе - next week
• Месяц. В + Prepositional case: в январе - in January, в этом месяце - this month , в прошлом апреле - last April
• Years: В + Prepositional case: в 2025 году, в следующем году - next year, в девяностых - in the 90s BUT: we can also say в девяностые - during the 90ы, в год моего приезда - at the year of my arrival, в трудные годы - during the difficult years - Accusative case.
• Centuries: В + Prepositional case: в прошлом веке - last century, в 21 веке - in the XXI century, в следующем столетии - next century. Век и столетие is the same.

BUT:

• в Средние века - in the Middle Ages.
• В век технологий - in the age of technology - Accusative case.

Other fixed expressions: в детстве, в отпуске, на каникулах.

• In childhood, on vacation, during the school holidays.

When do we use в девяностых - в девяностые?
There's some little nuance between the Accusative and the Prepositional.
So we use в девяностых when we talk about time as a container, a background, often it's more formal or descriptive. Lime, meaning: During the 1990s (as a historical setting, background)

For example:

• В девяностых годах экономика страны переживала кризис. In the 90s, the country s economy went through a crisis.
• And в девяностые в with Accusative case - is used when time is seen as a specific period of action - it's more conversational. В девяностые годы я был ребёнком - I was a child in the 90s. Focus is on what happened during that time, we don't just describe the time but we want to focus on what happened then. And it's very common in spoken and neutral language.

But again, they are interchangeable.
Look at some related patterns:

• В прошлом году or в прошлом веке - the Prepositional case, the norm, during that period.
• В прошлом веке люди писали письма от руки. - In the last century, people wrote letters by hand.

And these are exceptions: в год моего приезда, в трудные годы, в век технологий, в средние века. What's the difference here, what's; the nuance? We use the Accusative here, because we're talking about "at the time of, a historical period or a symbolic period of time".

• В век технологий всё меняется быстро. At the age of technology, everything changes quickly.
• Люди верили в магию в средние века. People believed in magic in the Middle Ages.

This is the logic: when the expression covers several centuries or eras, we use plural + Accusative case to show a broad historical span.
And when we talk about a specific moment or symbolic time, we use the Accusative case in singular. But when we talk about time as a general period or a "container" of time, we use the Prepositional case.

The Instrumental case is used for parts of the day - время суток - and seasons - время года. It answers the same question Когда?, but gives a broader context.
- Утром - in the morning, вечером - in the evening, зимой - in winter, and so on. Let's think of it as the background on which events happen. It describes when something happens as part of the rhythm of daily life and seasons. We don' t use any prepositions here, no в, no на, no nothing, just the case itself.
So, время суток: днём - during the day, вечером - in the evening, ночью - at night. Время года: зимой - in winter, весной - in spring, летом - in summer, осенью - in autumn.
So when you say Вечером я читаю, it literally means I read by the evening time. There are no exceptions here, which is great!

The Genitive case is a tool for expressing exact precise calendar dates. When you need to specify the exact day of the month - it's like pointing to a specific square on the calendar.
For example:

• первого мая - On the first of May,
• седьмого марта - On the seventh of March,
• тридцать первого декабря. - On the thirty-first of December.

This is when you specify a day of the month, not just a weekday or time period.
This form literally means "of the first of May", and it s always used with ordinal numbers - первого, седьмого - without a preposition.
You can also use the same pattern to specify years:

• первого мая 2024-ого года - on the first of May, 2024.

So, when you add the year, the Genitive marks both the day (первого мая) and the year (2024-го года). Again, no exceptions.

So now you can confidently answer the question Когда? in Russian. Remember the metaphor: Accusative - the moment, Prepositional - the period or the container, Instrumental - the background, time of the day or season, Genitive - the date, the calendar. The more you think in these images, the easier it becomes to choose the right case automatically. And of course, Russian loves exceptions! These are set phrases, just memorise them as they are.
Большое спасибо! Пока-пока!