Best friend is not coming to my birthday party and is instead hanging out with his gf

Over reacting a bit.
Harsh facts are - people tend to care less and less about friends birthdays over time, your birthday is 4th of Jan so you will always be inviting and chasing people who are already tired from Christmas and new year, and finally, you've gave them barely 48 hours notice so you can't be massively shocked someone already has plans, why did you leave it so late?

It's my fault I left it so late but the point I am making is that my best friend is not attending.
Sure, he planned everything before even knew about my birthday, but is that something more important than a best friends birthday? Something that occurs once a year? It's just an one day excursion, they have done and will do this a thousand times a year. It's not a trip to another country where they booked something etc.
I am not sure if my feelings are normal hence why I am posting here.

This is what I'm saying, I think you're overreacting and likewise you could have a party any day, but you want it on THAT day.
I get you're disappointed but at 2 days notice I think expecting him to have either kept the date free or cancel/change his plans is unrealistic, nor is it very fair to make assumptions about what his priorities should be or what this trip also means to him and his girlfriend. I wouldn't take it personally and would chalk it up to bad planning.
Assuming all the guests are getting the same 2 day notice, are they all attending?

My birthday is this Saturday.
My best friend of many years planned an one day excursion (they leave in the morning and come back late at night) with his girlfriend and one more couple on the same day as my birthday. Note, he planned this BEFORE he even knew about my party.
Today I told him I will be hosting a party on the same day, because that's when my birthday is (many people invited), and he told me he won't be able to attend because he has an one day close trip planned (as I said it's somewhere very close, like one hour drive, and they won't be staying overnight, just spend the day there).
I can't help myself but feel a bit **** about it. Sure it was pre-planned but man, it's your best friend's birthday, it happens once a year. Excursions can happen literally every week. It's not like a big trip was planned, they didn't book any hotels etc etc.
However I am not sure if I am overreacting but it kind of made me feel a bit **** inside.
What are your thoughts? Would be understanding of it?

Only 2 out of the 9 are not attending, my best friend and one more person.
I feel like if I were in his shoes I would cancel it because a best friend's (not just a friend) birthday is once a year, me hanging out with my girlfriend or friends can happen anytime, any day, any week, any month.
Of course if they had hotels planned and all, or some very unique trip..sure. But they do these pretty frequently.
I guess it's a matter of priorities.

First and foremost Happy Birthday.
I think you’re overreacting. Personally it’s a who gives a sheet. A pre planned excursion with his girlfriend is just seen to him more important. I don’t know what this excursion thing is or means but the point and case I’m trying to make is, is a day with his misses is more of a priority than having to be in attendance for your birthday. Can never miss out time with your other for something like birthdays that people can make up for a day or two later.

"a day with his misses is more of a priority than having to be in attendance for your birthday"
Bold statement my man. Not saying it's incorrect, but if it's true, then what's the point? Sorry if I sound desperate or needy but my best friend is also a high priority, not only my girlfriend. Probably lower than a girlfriend, sure, but a high enough priority that a regular excursion that has happened and will happened a thousand times throughout the year is less important than a birthday that happens once a year. That's how I see it really.
I guess it's what you said, a matter of priorities. Time to reconsider things I guess.

Only 2 out of the 9 are not attending, my best friend and one more person.
I feel like if I were in his shoes I would cancel it because a best friend's (not just a friend) birthday is once a year, me hanging out with my girlfriend or friends can happen anytime, any day, any week, any month.
Of course if they had hotels planned and all, or some very unique trip..sure. But they do these pretty frequently.
I guess it's a matter of priorities.

You keep repeating the same issues - birthday/best friend/you don't think their trip is a priority, we get those. You keep saying he can choose to do something anytime, but so can you? you're choosing to make a short term plan that is sure to inconvenience people
I still think 48 hours isn't reasonable notice and it definitely isn't enough notice to get annoyed at someone having other plans over or fair to expect them to cancel due to essentially your poor planning. Cancelling a planned trip at so short notice would prob cause some tension with a girlfriend and if a friend was trying to force me into that position I might get a bit annoyed.
If it was me I likely wouldn't cancel my plans and my best friend wouldn't expect me to, likewise if I said 2 days in advance I'm throwing a party I'd have a lot of people say no/maybe, and I'd know that was my own fault.
Lastly, to be a bit more direct and rude about it, what's so great about this party that people should cancel other plans? Is it going to be dozens of people I know, is there food laid on, lots of booze? some sort of theme that you've made an effort over? Is this party going to give me proper 'fear of missing' out vibes or will it end up being half a dozen teenagers around a youtube screen? which is pretty much the definition of something you could do anytime.