First of all, you do see how you became "needless and wantless" around the holiday issue, right? Don't do that next year. I understand how it is in healthcare, but just work either Xmas Eve or Xmas, not both, okay?
Second - it's not too late to plan a holiday get together on another day. My mom used to do "Grandma's Xmas" a week before or a few days after Xmas, as her adult children and grandkids were all busy with their own families on the day of. And a guy I knew through my BFF used to throw a big party every year on Xmas day evening. He called it "The Loser's Xmas" and held it for something like 30 years. All his friends and acquaintances who didn't have someplace to be on Xmas came. I went with her the first Xmas that I didn't have my kids with me (after my Xmas Eve second date with Spa Guy) and it was a fantastic gathering. My sister has a big gingerbread house making party every year before Xmas and all her friends love it. Throw a potluck Xmas party sometime before Xmas (to save yourself some of the work and expense) and invite lots of people. You're not the only one who is lonely around the holidays.
As for the SAD - make sure your vitamin D levels are good (this plays a huge role) and get a light box if you don't already have one.
