The scenario of this phrase is about a guy and a girl who are friends. The guy has feelings for her and vise versa, but the girl rejected the guy cause maybe she wasn't ready or she was confused. After the guy has moved on and has a new girlfriend, the girl finally admitted her true feelings to him but it's too late and thus, she's his Day Late Friend.
Jake and Mindy were best friends and they began to have feelings for each other. Jake tried to ask Mindy out, but Mindy rejected Jake even though she feels the same way. Jake moves on and finds another girl and they both got closer. By the time Mindy decided to express her true feelings to Jake, it's too late cause Jake has already moved on and is attached to the new girl and thus, Mindy is Jake's day late friend.