No experience yet but my son and daughter are 2 years 5 days apart. Both born in Feb one on the 13th one on the 18th!
Not planned (I'd had a miscarriage before my daughter so would have had an October baby) but it just worked out I conceived basically the same time as her brother with her.
We've decided that they will have a half birthday treat in the summer when the weather is nicer by otherwise unsure just yet as to how we're"" do it. It's my son's 3rd birthday coming up and my daughters 1st so we are going to just have family over and they can have a small cake each