I’ve written, “Hi Mrs. ___! This card will probably reach you Monday, so I hope that you had a wonderful birthday on Friday.”
I’ve been dating her son for 3 years this Tuesday. They are also taking me to Disney World with their family (and his brother’s friend as well) in June, a family member’s wedding in August, and the accompanying bridal shower next weekend. She has sent me at least 3 care packages since I’ve been at college, too…so this should be as much of a thank you note as a happy birthday note!
(I’ve read a few questions like this already…I’m not going to thank her for her son; it’s creepy so no need to suggest that!)
Perhaps something like:
“You’ve done so much for me I wish I was there to celebrate your birthday with you. I’ll be thinking of you on your special day and sending you well-wishings.”
I dunno, that’s not very good but maybe it will inspire you. Good luck.