There are 365.242199 days in a year
we round this down to 365 per year otherwise new years day would start 6 hours after midnight new years eve (which would probably be quite popular
).
of course this means we have 52 weeks and one day in a common year (non-leap year)
but if we dont adjust for the fractional day occasionaly then seasons are going to move later and later into the year
Julius Caesar kinda sorted this out and introduced the leap year every four years (the Julian calendar), but now our average year was 365.25 years long (longer than a real seasonal year) so seasons slowly started earlier and ealier in the year
Pope Gregory XIII kinda sorted this out and authorised a change to the leap year rule, where years that are divisible evenly by 100 must also be divisible by 400 in order for them to be leap years - 2000 was a leap year, 2100 will not be a leap year.
so our average year is now 365.2425 days long, even closer to a real seasonal year (but small adjustments are still needed)
When we moved from the Julian to the Gregorian calendar, the seaons had got out of wack by about 10 days (they were starting to early in the year), therefore 10 days were skipped out of the calendar: the days between and including October 5th 1582 (what would have been a friday) and October 14th 1582 never happened (great scotts!). So on thursday the 4th October 1582, everyone looked at their calendar and found that tomorrow was friday the 15th of October 1582.
