gayet wrote:Gives you time to "fix" them if needed.
I guess that's one reason.

The other reason is that it allows for people who started a two week walk on the 30th of the month to get home and still submit that great photo they took two weeks ago on the first day of their walk.
I've just added a note to the competition front page stating, "Note that monthly competitions are open for entries from the 20th of the month to the 19th of the following month."