February, the last month of the year was shorter because the year isn’t long enough to give it 30 days
Originally February had 30 days, along with all the other months. (The 5 or 6 remaining days at various times were either extra days which didn't belong to a month or were omitted until there was a month's worth of them to catch up on.)
February got shorter because (being the last month of the year) it had days removed in order to add them to other months -- what was originally February 30th (the end of the year) became February 23rd (the end of the year, after which the leap day is inserted).