Mon. is the short written form of Monday. You will see it on calendars, school timetables, shop signs, and at the top of letters or emails, where there is little space: Mon. 12 May. People almost only use it in writing. When we speak, we say Monday, not Mon. In British English there is often a full stop (Mon.); American writers often leave the stop off. It is not a different word from Monday, and it is not short for month. Though actually, which day counts as the first day of the week is not the same everywhere: many diaries in English start the row with Monday, but some (especially American ones) still start with Sunday, so Mon. may sit in a different column depending on the page.