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.
French Republican calendar
and:
These two specifications are incompatible, as leap years defined by the autumnal equinox in Paris do not recur on a regular four-year schedule. Thus, the years III, VII, and XI were observed as leap years, and the years XV and XX were also planned as such, even though they were five years apart.
A fixed arithmetic rule for determining leap years was proposed in the name of the Committee of Public Education by Gilbert Romme on 19 Floréal An III (8 May 1795). The proposed rule was to determine leap years by applying the rules of the Gregorian calendar to the years of the French Republic (years IV, VIII, XII, etc. were to be leap years) except that year 4000 (the last year of ten 400-year periods) should be a common year instead of a leap year. Shortly thereafter, he was sentenced to the guillotine, and his proposal was never adopted and the original astronomical rule continued, which excluded any other fixed arithmetic rule. The proposal was intended to avoid uncertain future leap years caused by the inaccurate astronomical knowledge of the 1790s (even today, this statement is still valid due to the uncertainty in ΔT). In particular, the committee noted that the autumnal equinox of year 144 was predicted to occur at 11:59:40 pm local apparent time in Paris, which was closer to midnight than its inherent 3 to 4 minute uncertainty.