Sat. is the written short form of Saturday, the day after Friday. You will see it on calendars, in diaries, shop hours, and timetables: “Mon. to Sat. 9–6.” In American writing the little dot (period) is common: Sat. In some British writing you may see Sat without the dot. It is for the page, not for careful speech — when you talk, you say “Saturday.” It is not a different word; it is just Saturday made short in writing.