We celebrate half-birthdays in our family with a cake and song (with our children all quite young, saying "I'm three-and-a-half" is soooo much older than simply being three, apparently). When our identical twin boys had their half birthday in June this year, I made a half/half cake and they each decorated their half.
** Made for the DSD Bingo Sketch challenge **
A Birthday Wish by Studio Flergs
https://www.sweetshoppedesigns.com/sweetshoppe/product.php?productid=33760&cat=&page=1