Dear …
Thank you for sharing in the celebration of my __(#)__ birthday! And thank you so much for the __(type of gift)__. I know it’s taken me a while to send out this note to you, nevertheless, I wanted to make sure that you know how much I appreciated your presence at my party and your gift.
I am so delighted that you came and hope you enjoyed yourself as much as I did having you there. Thanks for making my special day extra special for me by being part of it, __(Name)__. It was such a memorable birthday because of all who were there!
Best wishes,