There are six categories you can fill out, one of them being specific calendar days off. You can then rank order those six categories based on importance to you for that month. The categories are: Flight rotation length, Specific days off, Specific days on, # Days worked (range), Week days on, and Week days off.

I don't use those last two as I no longer have kids at home so just leave them blank and ranked 5th and 6th in my bid.