The Best Day Ever is both the greatest yet most annoying song ever made by Tom Kenny, Spongebob Squarepants voice actor.

The ear-raping tune was made for the Spongebob Squarepants movie (The first one). It was also used in a episode with the annoying little yellow asshole singing the song while riding a huge Krabby Patty.
Person 1: What song so you wanna hear?

Person 2: The Best Day Ever by Spongebob Squarepants.

Person 1: Da faq you say bitch?
by Send Cat Pics February 13, 2018
The day that never seems to exist but only as a mere figment of your imagination.
Child: Today is going to be the best day ever!

Also Child The Next Day: Today is going to be the best day ever!
by TheWarHorse100 November 30, 2019
Phrase you can blurt out when something good happens to you. In meaning combines "thank you," "how nice of you," "I really appreciate it," and "awesome" all into one expression. Meant to make the other party feel that something they did was well received.
Scene: Aaron is sitting on the couch watching TV and Wendy brings him the remote and a bowl of ice cream.

Aaron blurts out: "best day ever!"
by perthguy1 December 15, 2010
The best day ever known to man is February 11th!! If you were born on this day your are everything great!
Bob: Omg this is the best day ever!!

Joe: that’s because your with me and I’m born on February 11th
by 1great2awesome_ October 21, 2019