Well, for me I set a date, so that people know no matter which day it falls on, that after that date they can see new instalments... and it works as I also tie it in with the newsletter.

When I had a paysite version happening, I kept it to the 7th, 14th, 21st, and 28th... so there were 4 updates per month, irrespective of when those dates fell, this allowed for people to know, 1. that there were regular updates, 2. when to expect them and adjust accordingly.

I think the problem with choosing a 'day' vs 'date' is simply it does add a sense of unfairness in the customer's eye. But if you promote it, that every 'x' day of the month or 'day' of the week, you'll find that most members will return shortly afterwards, it gives them a way to feel excited, to set a schedule and when tied it with a newsletter, works real good.

just my thoughts on it but it worked for me.
Ian