itch does let you set an item to Completely Free, i.e. you literally can't pay any money, which is what i'm assuming the calendar will be set as (they said it'd be "print and play format" so i'm imagining it as a single item with all the content aggregated and not an itch.io bundle), but it's good to clarify!!
OH okay I see, I assumed it would be a co-op bundle, not a unique release. I don't think you can set those to not accept payment, but I've never tried.
In fact, we like explaining to people that if they do pay for it at all (including $1), that claims each title to your account. So like if I go to one of the item's Itch pages, it'll show me at the top that I own it, which is good cuz I often forget what I own lol.
We've done a few PWYW (pay what you want) bundles like this and they usually make more money than our paid bundles do 🤣