How Many Days Are in a Year? (365 or 366)
Most years have 365 days, but one in roughly every four has 366 — an extra day added as 29 February. Which kind of year you are in changes more than the date: it shifts day-of-year numbers, nudges your birthday to a new weekday, and affects any deadline measured from the start of the year.
365 days in a common year
A common (non-leap) year has exactly 365 days. Divide that by seven and you get 52 weeks with one day left over. That single leftover day is why the calendar does not repeat neatly: a date that falls on a Monday this year lands on a Tuesday next year, and jumps two weekdays across a leap year. You can check the weekday of any date with the Day of the Week Calculator.
366 days in a leap year
Roughly every four years, a leap year adds 29 February and runs to 366 days. The rule is not simply "every fourth year" — century years are skipped unless they divide by 400, which is why 2000 was a leap year but 1900 and 2100 are not. The full logic is set out in what is a leap year, and you can test any year instantly with the Leap Year Calculator.
Weeks and the leftover days
In numbers: 365 = 52 weeks + 1 day, and 366 = 52 weeks + 2 days. Those leftover days are the reason a calendar year never lines up perfectly with a whole number of weeks — and the reason some years contain 53 numbered weeks rather than 52. That week angle is covered in how many weeks are in a year.
Counting days within a year
Every date has a day-of-year number from 1 to 365 (or 366), and it is often useful to know how many days have passed or remain. For any span — from the start of the year to today, or between any two dates — the Days Between Dates calculator gives the exact figure, handling the leap day for you.