What is the term for two siblings born on the same calendar day, one or more years apart? I know there's a term, and I'd know it if I saw it, but I can't for the life of me remember it.
Aside from saying that you "share a birthday", I am not sure that such a term exists. If it does it isn't likely to be very well known. Restricting it to only match siblings is likely to make things less likely to match up perfectly.
Generally, special birthdays are given a modifier. Most of these are just off the cuff:
- holiday birthday
- shared birthday
- golden birthday
Other words that could work apart from "shared":
- joint birthday
- combined birthday
You could also go the other way with it and accentuate the negative:
- split birthday
Are you thinking of "Irish twins"? It's not exactly what you were asking about, but it is the only relevant term I can think of that you might be referring to.
Irish twins are siblings who are close enough in age to be mistaken for twins. I would guess that the expression stems from a perception that Catholics have children in close succession.
I might call them "yearlings", being a little cheeky.
Although the common usage is a foal between one and two years old, apparently there is a legit usage relating to a one year span:
Yearling (adjective): of a year's duration or standing
Having a sibling born 2 years to the day from me, I have searched for this term most of my life, but not found one. As such, and as language is merely an expression of randomly used phrases that come into popular acceptance, I decided to make up my own. As a play on the term "Irish Twin" for a sibling born in the same year, I refer to mine as my "Double Dutch twin".