I've had a festival with no specific name in my story for as long as it has existed. It was a harvest festival originally, which developed a strong emphasis on thanksgiving and rejoicing.
But now I'm tying to it to a specific historic event. It is also to celebrate the sighing of a charter/treaty which gave the chiefdoms of Verlis and Amray freedom to govern themselves separately from the old northern empire. Harvest festivals happen pretty much everywhere, so there must have been something before this happening.

The trouble is I have no idea what to call it. I've been using Latin and Old English as my source languages for creating names (if I don't just make them up), but I didn't find anything Latin based that I liked.
It's a big important festival held in every large town, and referenced many times through the early part of the book, so it needs a name.
The people of the Chiefdoms take this part of history pretty seriously too. They commonly count their years from when this document was signed.

What were the names of some of the historical figures involved in the original treaty singing? Was any one particular diplomat credited with fostering the treaty? That is, do you have a George Washington-type character that your country might be drawn to?

Did you look up the Latin and Old English names for freedom, etc?

I think it's important to remember that festival days do not always have to be that creative. Most American holidays really aren't that originally named. Independence Day is pretty, well, straight forward. Even words like Easter and Christmas have fairly obvious origins. So if this is a holiday celebrating freedom and what's essentially the beginning of a new country, then it would probably be named something fairly obvious like Freedom Day.

Of course, if you use patron saints or similar you could call it "Saint Amaruella's Day"; since she's the saint of self-rule and double chocolate fudge.

Or just "Treaty Day", but sales of fudge would be down...

Greek, Greek, It shouldn't exist in the world, but on searching, I realize the name I use for the world, Eleftheri, is transliterated Greek for 'free'. I don't use the name within the story, or in anything official, but is there in my head. So maybe I can use it. Or I could go with Old English.
freólsbóc or bócriht for charter.
fréolstíd for festival.
Those just happen to have the segment in them which is interesting.
foreweard or getreówþ for treaty.

I don't know what individuals were involved. I've not looked that much into that part of the history.

I did have in my head, that the festival continues over several days. One day could especially be Charter day.

I'll have to think on it a bit more later. I don't have enough brain power right now.

My first thoughts are "Sovereignty" or "Separation" ("comma-Feast-of").

Those are both really good ideas.

Playing with Old English is another fascinating option. Lots of choices there, especially if you do some research into Anglo-Saxon society and what their festivals included and how they were named.