MST has 2 meanings
Must see today
A quick way of saying, "You've gotta see this today," as with something that can't wait until tomorrow; sometimes used to refer to movies or items for sale in classifieds or advertisements.
The film critics reviews say it's a MST movie.
A time keeping acronym that refers to the Mountain time zone in the U.S. during fall and winter; mostly made up of the Rocky Mountains; ahead of the Pacific Time Zone by 1 hour, behind the Central Time Zone by 1 hour, and behind the Eastern Time Zone by 2 hours.