Common year
There are 7 months with 31 days. There are 4 months with 30 days. There is 1 month with 28 days. In the Julian calendar, introduced by Julius Caesar in 45 BC, every 4 years there are 29 days in February, which is called a leap year. This happens because every year is 365 and 1/4 days but instead of us having a spare quarter of a day in each year we add them all up every 4 years and make an extra day to avoid confusion and make things easier for everyone.