One time my DS and I officially "swapped" birthdays. I'm in August when we were there, and he's in cold, dark January. He was very ill, and we didn't know if we'd still have him with us when his birthday came. So I "gave" him my birthday, he got all the lovely attention, and we had a wonderful time on "his" vacation birthday.
His condition got better, and he was out of the woods by his "real" birthday, so we celebrated mine then.
Unusual circumstances, but it worked for us, and there really WAS a birthday, so it didn't teach him that it's okay just to make it up.