In french, we say "le jour J" (literally "the D Day") to mean "the day where the thing happens" without knowing the actual date, J is like a variable here. Then

In french, we say "le jour J" (literally "the D Day") to mean "the day where the thing happens" without knowing the actual date, J is like a variable here. Then you can talk about "le jour J-1", "J-2"...