I could probably safely place this story happening between the breakup and New Years Eve (28th or 29th), Sarah could have come up with the idea of making MV5 as a way to get her mind off of the breakup, which goes with her reaction Ellen's sister comment. It can't be the 30th because Ellen and Elliot were together discussing Susan's birthday and when Grace had called Ellen, she looked as if she was in "just gonna chill around the house" mode and while we don't really know how much time pasted during this NP, it was probably longer than "Playing with Dolls" especially if you account for them playing video games afterwards.
It also couldn't have been New Years even or New Years day cus, well parties and monsters and burnouts and such. January 2nd would be out too because of Ellen and Nanase getting together to discuss what they learned the previous couple days, as well as play with magic. Beyond that we get into Elliot Sarah and Susan working on the video revealing the breakup and then school stuffs.