Freddie's date with Melanie was on Saturday (as said by Freddie when he asks her out). Since he later refers to the date as "last night", the final scene should take place on Sunday; however, Mr. Chambers comes by on the same day to tell Carly that Chuck failed his math test, which he is supposed to take on Monday.
Carly hits the up button on the elevator call station, but she and Melanie are going down.