Perhaps you have to be "old" like we are or a "retro" Carpenters fan to get the reference in that title. It's a song lyric. "It seemes like rainy days and Mondays always get me down".
The Monday reference - well yeah. The weekend is over, your unfinished weekend projects are still calling to you but you need to switch gears and dig in to work.

If you don't have a job (and need one) just about ANY day could get you down for all the worry and stress that comes with being out of work. If you are retired - or for the rare soul out there who does not need to be part of the weekly work force - just about any day can be Monday for you. Good, bad or otherwise. Mondays at work though mean that you HAVE a job, and you are paying the bills. Without the job - all those fun things are no longer "fun" if you're worrying about paying your bills.
See. Perspective. That's all.
If your projects or hobbies are the out of doors kind, then they could get postponed for rain. On the other hand, rain fills up the reservoirs (for you to ski or go fishing in this summer - and drink from all year), waters the lawn for you, and cleans the air. Spring rains seem to help with the pollen allergy issues too. Rain also means it's not going to freeze, because as we know, freezing rain is more along the lines of ice, snow or hail. Your seedlings in the garden are getting watered (hopefully not flooded as in the case with a whole BUNCH of rain at a whack) and the countryside comes alive with wildflowers.

A rainy day is often a good excuse to stay inside and get to some of those projects we have been meaning to do and just didn't have time for because of being busy outside. I think that's one of the things that make Winter one of my favorite times of year. My hobbies are mostly indoors kind of things, most of which I can indulge in even on a rainy day. The exception sometimes being when the power goes out.
Have you ever been home on a rainy day and the power goes out, and you think to yourself -' hmmmm....leeme think....what can I do now that the power's out and I can't watch TV? I know, I'll vacuum! Oops...need power for that too. OK...how about if I sew?' Unless you're in to treadles and hand-cranks or hand sewing, or knitting
- that one is not going to work either.

It was such a pleasing sight to watch the hummingbirds flit through it as each day the dried sticks of the crepe myrtle came a little more alive with tender green leaves. Notice, I said WAS a pleasing sight?
April has traditionally been a month where, at least in the places I've lived in California, we got one week of each season. This month, we had Spring week and Summer week the first couple of weeks, and the crepe myrtles got fooled into thinking it was safe to sprout leaves.

Apparently, last week was "Winter" week...and now...those pretty green leaves are just dry black things dangling there all morose.
I guess is "Fall week" because we've had rain, brisk cool days, and a bit of wind. The hummingbirds care little for what season it is - so long as they have full feeders. They are fine with black or green leaves on the crepe myrtle too.
So, here's hoping your rainy days and Mondays are all good. If if they do get you down occasionally, just hang in there.
They have their purpose, and usually there's a benefit just around the corner.
Meanwhile...