Days in a year
There are 365 days in a regular year and 366 days in a leap year in the Gregorian calendar. The average is 365.2425 days.
Although many other different calendars exist, the Gregorian calendar, a solar calendar, is the most accepted calendar around the world.
A day is defined as the approximate period of time taken for the Earth to fully rotate once around its axis, or 24 hours. A solar year is the amount of time that it takes the Earth to complete its orbit around the sun once. These definitions are what the Gregorian calendar is based on. The measured average solar year was 365.2421897 days or 365 days, 5 hours, 48 minutes, 45.19 seconds on January 1, 2000. The average of 365.2425 days in the Gregorian calendar is very close to the measured value. Overall, the rotation of the Earth is slightly irregular and is slowing down.
Leap years
A leap year is a year that has an extra day. In the Gregorian calendar, a leap year has 366 days rather than 365 days.
Leap years exist to keep calendars synchronized with whatever event they track. A year is the amount of time it takes the Earth to orbit the sun, but this number is not a whole number, so a day needs to be added periodically. A year is 365.2425 days in the Gregorian calendar, so every 4 years, one day is added to keep the calendar and the actual orbit in sync.
The extra day is added on February 29th of every year that is an integer multiple of 4 that is not evenly divisible by 100. Years that are evenly divisible by 100, except those that are also evenly divisible by 400, are not leap years. For example, 2023 is not a leap year since it is not an integer multiple of 4. 2024 is a leap year since it is an integer multiple of 4 and it not evenly divisible by 100. 2100 is not a leap year since it is evenly divisible by 100 but not evenly divisible by 400. 2000 is a leap year since it is evenly divisible by 100 and is also evenly divisible by 400.