From ThresholdRPG Wiki

Monday Night Flagging is an event that takes place every Monday on Threshold. Every character logged on at 8pm EST/EDT is flagged when a global announcement goes out. If you are still logged on 3 hours later you receive a bunch of rewards; usually a big chunk of XP towards your next level, a bunch of useful items, and some buffs.

Staff and players very often plan events and major announcements during MNF because there are usually a lot of people online at this time.

Being online during MNF is a great way to meet and interact with a lot of characters. It is a great event for players old, new, or anywhere in between.

The purpose of this event is to promote roleplay by getting people online and active at the same time so "things can happen."