It is terribly sad and it totally sucks to face these special days.
Remember, if you were to stay with your WH, there would be even more sad occasions: new d-days. He is a cheater and he lacks the character and remorse to change. It's only a matter of time before he cheats on his OW because he lacks good boundaries and solid morals.
Plan something special for Thanksgiving, Christmas and New Years. Start a new tradition with your son, get together with relatives, find a new dog friendly park to take your furbabies, get tickets to a show or a game . . . make sure you have things to look forward to.
Are you in IC? Even just a few visits was really helpful for me.
You are not weak. You are dealing valiantly with betrayal--one of the hardest things a person can face. Keep going. Hug your son who is so lucky to have you for a mom. Someday, you will be happy and thriving. You will find a man who knows how to be faithful and is worthy of you.