In the Gregorian calendar, which is the most widely used calendar in the world, there are 12 months in a year. Seven of these months have 31 days: January, March, May, July, August, October, and December. The remaining five months have either 30 or 28 days, with February being the shortest month with 28 or 29 days depending on whether it is a leap year.
The reason why some months have 31 days and others do not is a bit complicated and has to do with the history of the calendar. The Gregorian calendar is based on the solar year, which is the time it takes for the Earth to orbit the Sun. One solar year is about 365.242 days. To make the calendar easier to use, we round this number to 365 days, which means that we have an extra 0.242 days each year. This extra time adds up over the years, and every four years we have an extra day, which is added to February as an extra day, making it a leap year with 29 days instead of 28.
Now that we know why some months have 31 days and others do not, let's take a closer look at each of the seven months that have 31 days.