Not only will this help you stay consistent, it will keep you from accidentally booking things that might conflict with it and then having to cancel your writing session.
Once you decide which days you’re going to write, what time you’re going to write and for how long you’re going to write each day, all of that should go into your calendar as an appointment. Not only will this help you stay consistent, it will keep you from accidentally booking things that might conflict with it and then having to cancel your writing session.