Same with Animal Crossing.  
Toy Day = Christmas  
Harvest Festival = Thanksgiving  
Everything else is the same

isn’t St Patrick’s day the Clover Festival?

April Fool’s Day would be Celestial Fool’s Day. Princess Celestia reinstates an old holiday in which she essentially allows her subjects to prank her, reminding them that she is just a pony like any other. The holiday turns hostile, however, as the pranks gradually turn into abuse. Celestia then reveals her own prank, which was announcing the holiday one day early, in an effort to curb the violence. Everypony freaks out, thinking the holiday was just an excuse to banish everyone, but Twilight stands up for Celestia, preaching the holiday’s message that even rulers are human/equine. Celestia then cancels this year’s Celestial Fool’s Day.

“So what are they gonna do for Easter,”
 
Winter Wrap-Up

‘cause there ain’t nothing offensive about wood

@Dr_Osaka  
Easter: Zombie Pony Jesus day.  
Thanksgiving: Doesn’t exist because Thanksgiving isn’t a commercially viable holiday.  
April Fools: April Foals.  
St. Patrick’s Day: St. Uh… I got nothing.

Shouldn’t Nightmare Night be on the winter solstice?

So what are they gonna do for Easter, Thanksgiving, April Fools and St. Patrick’s Day?

They spell arbor with an E.  
It’s a huge difference.

Maybe because it’s the only of the bunch that doesn’t have religious influence.  
Or maybe because it’s not really that big of a holiday to change it.