Whatever whenever, basically. I do have different ideas and follow the progression of them, but nothing that resembles a certain weekday being festival day for example.
I'm admittedly not sure how strict you mean it, though. I guess there is pretty well always some sort of scheduling now that the calendar is base game. I do indeed schedule some events. Curfews from Parenthood might count as well, and I do use them but more fictitiously.
I also have technically waited until day time or night time to do this or that (usually around a Sim's career) or even for an off day it was an all day activity. However, this is usually more... An obligation than a choice. Needs have to be tended or time sensitive obstacles have to be worked around, etc. *shrug* I wouldn't call it scheduled, but maybe you would.