The Soviet calendar was a calendar used in the former Soviet Union (now many different countries of Europe and Asia). It had different months and days of the week, and it was very different from the Gregorian calendar used in most other countries.
The Soviet calendar started with a week set to seven days, but the days were not named. Instead each day of the week was given a number from 1 (Monday) to 7 (Sunday). The months were also named differently, and the length of the months was based on the seasons. February was always 28 days and the other months were either 30 or 31 days. The year was broken up into 4 seasons of 91 days each, which made 365 days in the year.
The Soviet calendar was used in the Soviet Union and in some other countries until the 1990s. Today, people in those countries generally use the Gregorian calendar and the days and months are named the same way as in the US and Europe.