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.
Thanksgiving (United States)
The date of Thanksgiving Day follows a 28-year cycle, broken only by century years that are not also a multiple of 400 (2100, 2200, 2300, 2500, …). The cycle break is an effect of the leap year algorithm, which dictates that such years are common years as an adjustment for the calendar-season alignment that leap years provide. Past and future dates of celebration include: