Made by: Manyhills
The original strip had Garfield remarking "Man, I should have been paying more attention" in the last panel. This strip changes both that line and slightly alters the other panels (note Jon saying "Today was a good day" instead of "This has been a good day") to create a reference to the final lines of "It Was a Good Day" by Ice Cube - "Today I didn't even have to use my AK/I got to say it was a good day".
Of note is the author's notes - an IRC conversation between Manyhills and someone named "p_k", where the former posts his idea for the strip and asks if it would not be funny. p_k says it wouldn't, then tells him to do it anyway. Thus, here we stand, reading the end product.
[20:59] <me> can i just check
[20:59] <me> editing http://images.ucomics.com/comics/ga/2012/ga120604.gif to have the dialogue
[21:00] <me> "today was a good day" "it was?" "man, i didn't even have to use my ak"
[21:00] <me> would not actually be funny, right
[21:06] <p_k> are you listening to ice cube
[21:07] <p_k> also no it wouldn't be funny
[21:07] <p_k> you should do it
Original strip: 2012-06-04.
Jon: Today was a good day.
Garfield: It was?
Garfield: Man, I didn't even have to use my AK.