No one knows how crowded the festival will be in 2021. Disney has created a new park reservation system and capacity is still not maxed out. But as far as I've seen, no one knows what that capacity limit currently is. If this year is like past years at all, weekdays will be less crowded than weekends. You can also check out reservation availability here to get an idea of whether Epcot will be at max capacity that day or not. And know that before the 50th celebration begins on October 1st will likely be less busy than after.