Sunday is my admin day. It's the day of the week I schedule a lighter load (usually) so I can have somewhat of a day of rest. Usually I spend the day tying up loose ends, dealing with paperwork and invoices and editing. It also gives me an opportunity to catch up on lower priority things from that week plus plan my schedule and team work for the next week.
Because freelance writing is often a 7 day a week job, it can feel like one day isn't much different from the next. You can set up specific days of the week for particular tasks. For example, if one client likes a weekly invoice, designating a day helps you remember and plan your week accordingly. If you try to keep on top of your own bookkeeping and other tasks, setting aside a day to organize everything once a week could do three or more things for you.
1. It helps you keep your workload manageable because you can stay organized.
2. A day with a bit less stress is nice as your workload is lighter that day.
3. Decluttering your desk and your e-mail inbox can do a lot for your productivity.