The idea is to avoid sending all weekly emails the same day (which would be bad on a large / multi-domain instance). 
This means, if a user subscribe to the weekly digest a monday, he will always get his email digest on monday. That's why I need the day of the week in the key : it allows building the exact key of the row containing all users that must receive a weekly digest today.

I can add a comment to make that clear if you want.