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
February is also the only month of the calendar that, once every six years and twice every 11 years consecutively, either back into the past or forward into the future, has four full 7-day weeks. In countries that start their week on a Monday, it occurs as part of a common year starting on Friday, in which February 1st is a Monday and the 28th is a Sunday; this occurred in 1965, 1971, 1982, 1993, 1999 and 2010, and occur will again in 2021. In countries that start their week on a Sunday, it occurs in a common year starting on Thursday, with the next occurrence in 2026, and previous occurrences in 1987, 1998, 2009 and 2015. The pattern is broken by a skipped leap year, but no leap year has been skipped since 1900 and no others will be skipped until 2100.