How to use prepositional case to define time? (Months)

Question

I am told to use propositional case with the names of the months to define time.

However, I am confused as I can't seem to find how. For example, how did Сентябрь becomes в Сентябре? Декабрь to в Декабре? Май to в Мае? Август to в Августе? Right?

May I know how to construct this? Or is it just simply change the endings into <<е>>?

Answer 1

1. If the month name ends with Й or Ь, remove the last letter.
2. Add the ending -е.
3. Add the preposition в.

Note that month names are not capitalised in Russian.

• Янва́рь - в январе́
• Май - в ма́е
• А́вгуст - в а́вгусте.

In январе́, феврале́, сентябре́, октябре́, ноябре́ and декабре́ the stress shifts to the ending. For other months it stays put.

Answer 2

Check out, for instance, following reference, where you can find a lot of information about Russian prepositional. Here's a quote:

Время и условие

Кроме пространственных значений предложный падеж может выражать значение Времени: в детстве, в мае, на прошлой неделе, на рассвете, при царе, при встрече, при каждом удобном случае, по приезде.

So, to answer your question, you use prepositional with moths when you saying when - so "в январе", "в феврале", "в марте" - and so on - luckily they all are inclined the same way - so indeed all end with "-e".