What is the historical background of the Buddhist Lent season or rainy retreat and the activities of monks during these months?

This period of the year is known as the Buddhist Lent season or rainy retreat. I would like to know the historical background of this religious event and the activities of monks during these months.

Kalmykia :- The only Buddhist republic in Europe

The president of Republic of Kalmykia gave a speech in V-Star day. He is impressed with this historic event and will introduce the V-star project to the youths in Kalmykia.