The song is from season 4 episode 12 of MLP:FiM.
♫ Lyrics for the song:
( You can support the show here: https://itunes.apple.com/us/tv-season/my-little-pony-friendship/id727332784 )
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[Pinkie Pie]

Get your hooves up, party's starting out right now
Everypony, everypony get down
Time to make a wish, better make it right now
It's been a year and today is your birthday party

Make a wish, it's your birthday
Make a wish, it's your birthday party
Make a wish, it's your birthday
Make a wish, it's your birthday party

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"My Little Pony" is owned by Hasbro,
"MLP: Friendship is Magic" is created by Lauren Faust.