How does it work at a regional when someone decides to join the Army Reserves or one of the other reserve branches and they're currently employed at a regional?

I understand they have to give you your job back with your seniority number when youre done with basic or if you were deployed but how would you go about mixing your pairing with the one weekend a month thing? I wondered if you had to do those on your days off or are you excused from flying a weekend a month to goto drills if the weekend falls on your line.