National Thank God It's Monday Day, TGIM, goes into the question mark bin. We're not sure why we celebrate the first Monday of the year. Perhaps it's to be different, since on every other week we typically thank God it's Friday (TGIF). If you find it difficult to get into the spirit of being thankful for the first Monday, perhaps you could tweak the celebration into National Thank God Monday's Over (TGMO) with your choice of spirited beverage at workday's end.
Another interpretation/speculation could be with the holiday break and New Year's Day potentially on a Monday, that many companies and schools have the first Monday of the year off as a holiday. Thus, this holiday could be celebrating Monday in a tongue-and-cheek way being a potential off day for many.
We're not sure who started this holiday and why, though, we'd like to find out. Personally, we prefer either the celebrating the first Monday because you don't have to work, or the Thank God Monday's Over (TGMO) version if you do work on that day.
In the event that you're one of those people who just loves to work, then this holiday is truly for you. If you'd like to be one of those people (or you're a manager who'd like their staff to have this mindset), then maybe the ThankGodItsMonday.com website can help.