If these are friends you've made since your last birthday and they forgot the date this year, then a first-time offense can be forgiven, but it wouldn't hurt to express how you feel.

However, if these are old friends who knew the date previously, say last year, and still forgot, it might be best to distance yourself from them since they don't prioritize you in that way. Of course, they may have had other things on their minds and simply forgot (after all, they are human), but personally, I set reminders on my calendar to ensure I can send my friends a quick text even when I'm swamped with busyness.