As others have stated above, scheduling of GUP downloads is not possible.
What other have not addressed is the Why. You see, GUPs will only download content if & when requested by a client. If no client requests updates for three days, the GUP will not download definitions for those three days. If one client requests delta updates between yesterday and today; that delta update is different to a delta update from the beginning of the week to today.
That means you will never know when a GUP will download definitions and how big they are. As discussed in other threads, if you want to prevent GUPs downloading updates during the morning rush, make sure there is a client that requests the latest updates from the GUP at least an hour before the morning rush.