Я записал в календарь, что седьмого ноября мы идём на концерт, а восьмого декабря встречаемся снова.

Записал is the past tense of записать, which means to write down, to note down, or to enter something.

Its form tells you a few things:

• past tense
• masculine singular
• perfective aspect

So the speaker is male, and the action is presented as completed: he made the note in the calendar.

Compare:

• я записал — I wrote it down / I entered it
• я записала — same, but spoken by a woman
• я записывал — I was writing it down / used to write it down / was in the process of noting it

Here, записал is natural because the note has already been made.

Because with записать Russian often treats the calendar as the destination of the information: you are entering something into it.

So:

• в календарь = into the calendar, as an entry
• в календаре = in the calendar, as a location

This is similar to the difference between:

• Я записал это в блокнот. — I wrote it down in/into my notebook.
• Это записано в блокноте. — It is written in the notebook.

In this sentence, the speaker is talking about the act of making the entry, so в календарь is the natural choice.

They are there for two different reasons.

1. Comma after календарь
   The word что introduces a subordinate clause:
   Я записал в календарь, что...
   = I wrote in the calendar that...

   In Russian, subordinate clauses are normally separated by commas.

2. Comma before а
   А connects two clauses:

• седьмого ноября мы идём на концерт
• восьмого декабря встречаемся снова

Since these are separate clauses, Russian uses a comma before а.

That is the normal Russian way to say a calendar date.

When Russian answers the question Какого числа? — On what date? — it commonly uses:

• the ordinal number in the genitive
• the month also in the genitive

So:

• седьмое ноября would be wrong here
• седьмого ноября = on the seventh of November
• восьмого декабря = on the eighth of December

You can think of it as historically related to an understood word like числа:

• седьмого (числа) ноября

Even if числа is not said, the genitive pattern remains.

Because Russian often uses a bare date expression for exact dates.

English usually says on November seventh, but Russian often just says:

• седьмого ноября
• восьмого декабря

No preposition is needed.

Compare:

• в ноябре — in November
• седьмого ноября — on the seventh of November

So for a whole month, в is common, but for an exact date, Russian very often uses the date by itself.

Because in Russian, names of months are normally written with a lowercase letter, not a capital.

So:

• ноябрь
• декабрь
• понедельник
• январь

They are capitalized only if they begin the sentence or are part of a title.

This is different from English, where November and December are always capitalized.

Russian often uses the present tense of an imperfective verb to talk about a planned or scheduled future action.

So:

• мы идём на концерт can mean we are going to a concert
• not necessarily we are going right now

This often sounds natural when the event is arranged or already decided.

Compare:

• Завтра мы идём в театр. — Tomorrow we’re going to the theater.
• Летом едем в Сочи. — We’re going to Sochi in the summer.

So идём here is a normal way to describe a future plan.

Because Russian normally uses на with events and activities.

So you say:

• на концерт — to a concert
• на лекцию — to a lecture
• на выставку — to an exhibition
• на работу — to work

By contrast, в is more typical with buildings, enclosed spaces, or institutions:

• в школу — to school
• в театр — to the theater building
• в музей — to the museum

A concert is treated as an event, so на концерт is the natural choice.

Встречаемся comes from встречаться, which often means to meet each other or to arrange to meet.

The -ся ending is important here because it gives the sense of a mutual action:

• мы встречаемся — we meet / we are meeting each other
• мы встречаем — we meet someone/something else

So the reflexive form is the correct one when two or more people are meeting one another.

Also, just like идём, this present-tense form can refer to a future arrangement:

• встречаемся снова — we’re meeting again

Because а often links two things by contrast or comparison, not just simple addition.

Here the sentence sets two dates side by side:

• седьмого ноября — one event
• восьмого декабря — another event

So а feels natural because it means something like:

• and meanwhile
• whereas
• while
• sometimes just a contrastive and

If you used и, it would sound more like a plain list of events.
With а, the speaker is more clearly contrasting one date with the other.

The word order is flexible, but this version is very natural.

Russian often moves information around to highlight what is most important. Here, the dates are prominent because this is something written in a calendar, so it makes sense to place them early:

• седьмого ноября мы идём на концерт
• восьмого декабря встречаемся снова

You could also hear other orders, for example:

• Мы идём на концерт седьмого ноября
• Мы снова встречаемся восьмого декабря

Those are possible, but the original order emphasizes the dates, which fits the calendar context very well.

Yes, it could be omitted if the context is clear, because Russian often drops subject pronouns.

For example:

• Я записал в календарь, что седьмого ноября идём на концерт...

This is possible if it is already obvious who идём refers to.

But including мы can make the sentence clearer and slightly more explicit. In longer sentences, Russian often keeps the pronoun to avoid ambiguity and to make the structure easier to follow.

So:

• with мы — clearer, more explicit
• without мы — also possible if context makes the subject obvious

Yes, there is a slight difference.

• снова = again, neutrally
• опять = again, but it can sometimes sound more emotional, repetitive, or even slightly annoyed, depending on context

So here:

• встречаемся снова sounds neutral: we’re meeting again
• встречаемся опять is possible, but in some contexts it can feel a bit like again...

That said, in everyday speech опять is very common and does not always sound negative. But снова is often the safer, more neutral choice in a sentence like this.