Leap year
The name "leap year" probably comes from the fact that while a fixed date in the Gregorian calendar normally advances one day of the week from one year to the next, the day of the week in the 12 months following the leap day (from March 1 through February 28 of the following year) will advance two days due to the extra day (thus "leaping over" one of the days in the week). For example, Christmas Day (December 25) fell on a Sunday in 2016, and Monday in 2017, then it fell on Tuesday in 2018, and will fall on Wednesday in 2019 but then "leaps" over Thursday to fall on a Friday in 2020.
February 29
The Gregorian calendar repeats itself every 400 years, which is exactly 20,871 weeks including 97 leap days (146,097 days). Over this period, February 29 falls on Sunday, Tuesday, and Thursday 13 times each; 14 times each on Friday and Saturday; and 15 times each on Monday and Wednesday. Excepting when a century mark that is not a multiple of 400 intervenes, consecutive leaps days fall in order Thursday, Tuesday, Sunday, Friday, Wednesday, Monday, and Saturday; then repeating with Thursday again.