Like Double Solitaire in Reverse?
It's pretty easy to celebrate Play a Multiplayer Game By Yourself Day. Virtually everyone owns a game. All you have to do is get it out, set it up, review the rules (if needed), and start to play.
There are many multiplayer games that have rules for solo play. Even the great game of Agricola has them.
Sometimes playing solo makes sense if you just got a new game that you don't know how to play yet. Teaching yourself before teaching someone else is usually a good idea.
Did you ever play Monopoly by yourself? I'm pretty sure I have. You probably played as if you were two or more players - not strictly solo as some games allow.
Did you win?
This reminds me of Gilligan playing tic-tac-toe by himself. If I recall correctly, he said he never won.
We currently have 3 new games sitting on the table in the living room - Pastiche, Tournay, and Fungi (aka Morels). I think I'll play Pastiche soon, either by myself or with someone else.
Let us know which game you decide to play by yourself today.