I can relate to this now that I'm in a steady Monday-Friday work routine (I'm on placement for college - it's like a full-time job but you don't get paid).
I think for me it's the stark contrast between having a routine during the week, and doing much less during the weekend. I think one way I could improve the "Sunday blues" is to have more of an established routine on the weekends.
What I currently do is on Monday I just throw myself back into the routine - wake up at 6:45am, shower, feed the cats, breakfast, walk to placement, work until 4. By the time I'm at work for a couple of hours, I'm back in the groove and ready for the week.

And honestly, I'm a lot happier when I'm engaged in a routine.
This post has given me a push to improve my weekend routine, so thank you very much for that.

I hope we both have a fabulous Monday.