Friends less invested in friendship than you are

I'm feeling hurt lately as my birthday is coming up in a few weeks and pretty much all of my friends (some I've known for 20+ years) have already made other plans on the date I usually celebrate. They will be away all week, so there's no chance I can do something on another date either.

I plan my social calendar around their birthdays to ensure I'm available, so it stings to learn they don't do the same. I'll admit I've gone quiet on them as they didn't even offer an apology but I'm struggling to move past it.

I suppose I'm just looking for some empathy/understanding. Does anyone else feel like their friends don't return the effort you make for them?