Always save the best for last, even if being last means your birthday greeting arrives a little later than intended.
I didn’t intend on being the last one; it was just one of those things that sort of happens without you really being able to explain it. Well, maybe a good way of putting it would be to simply say that I forgot and I am so sorry for not sending you these birthday wishes on time.
The timing may have been off, but the surprise factor was still there and you should really focus on the positive side.
I hope you had a fun day. Happy belated birthday!