For instance, the 225th anniversary of US independance coming up next year? I was just a little tyke when the bicentenial came and went, so I didn’t get to celebrate much and am looking for a possible excuse to do so now.
The only non-centeniary annivarsary I know for sure is the 150th, which is a sesquicentenial. Would a 250th be a bisesquicentenial? What of a 275th? And most urgently, the upcoming 225th? Is there a word for it?
The 250th anniversary is the bicenquinquagenary. Be sure to remember that 26 years from now.
The only thing I have found for 225 years is “225th anniversary”. Kinda dissapointing, actually. I’m sure their is a nice, fancy word for the 225th, I just don’t know what it is. I’s call the bride and groom loudly, though. They’re probably hard of hearing.
I nominate bicenquartial.
A long friggin time to be married
Well, there was that trial separation thing in the 1860s, but that just didn’t work out.
224 years and 37 kids later, not bad for a bunch of former Brits. 
Sesqui-sesquicentennial, perhaps? 1½ * 1½ * 100 = 225
I suppose really you’d want whetever the Latin phrase for “two and a quarter” is, with “centennial” after it. I never did learn my Latin numbers so well.
I found serveral sites about the 225th anniversaries of various things. They always refer to it as “the 225th anniversary.”
according to Allen and Greenough’s Latin Grammar site here, the adjective you might be looking for is bicenvicenquinquennial.
A couple of dictionaries give the definition of vicennial as ‘every twenty years’ and of quinquennial as ‘every five years’ with corresponding anniversary marks, and I kinda guessed as to the applicable roots for a complex ordinal number.
Kinda makes me wish I still knew that Latin teacher I used to date.