Keep in mind that she can appear; it's not a guarantee since her appearance is dependent on meteor showers. As per the datamines, she also can't appear on days that K.K. Slider is visiting on, so you can at least not worry about her on most Saturdays.
She does have some weird spawns, though, which makes her hard to find. I noticed that some visitors, like C.J. and Flick, tend to pick one location to spawn in for any given day and stay there regardless of how many times you exit the game. For example, the other day, I had C.J. spawn on a cliff with no ramp. He spent the entire day there despite exiting the game multiple times and using different player characters. It could just be a coincidence, but it seems like Celeste doesn't have that restriction. One day, I saw her walking around the plaza. I switched to another player character, only to find that she had relocated to a cliff without a ramp, so she couldn't have just walked there on her own.