What would you do with an extra second?! The possibilities are… limited. But that will be your opportunity tonight. The moon and redistribution of mass on Earth (earthquakes, moving glaciers, etc) effect the speed of rotation of the earth. Atomic clocks obviously aren’t affected by these things so for the same reason that we need leap days, we also need some smaller adjustments.
Wikipedia says this has happened 25 times since 1972 and tonight will be the 26th time. The International Earth Rotation and Reference Systems Service schedules these events six months in advance on an as needed basis.
I’ll spend my extra second sleeping… or more likely I’ll spend it trying to get my son to go back to sleep.