It should not be a whole day. The point of Community Day is to play together with the community. If you spread the spawns throughout the day, you'll meet less of your community.
For example, in my community if I farm dust on a normal Saturday/Sunday I see maybe 2 to 3 people who do the same. While on the same location, during Community Day, I see and have a chance to chat to many (50+) PoGo people.