TL;DR: Pick one set date early in the year and stick to it; accept that not everyone can make it every single time. In my experience, trying to accommodate everyone's schedule with a group of 20 is basically a recipe for a headache. Over the years, I have tried many different systems, from shared apps to massive email chains, but there is always someone who just cannot make it work due to travel or work constraints. I think the most reliable way to handle this is to set a permanent date, like the second Saturday in December, and just communicate it months in advance. Not sure but someone once told me that if you keep the date consistent year after year, people eventually plan their lives around it. For the uncle who doesnt use a smartphone, you could try sending a printed physical invitation or just a simple postcard in the mail. It sounds a bit old-school, but it keeps everyone in the loop without needing a tech-heavy solution. It is tough feeling guilty about empty chairs, but you really cant control where everyone else is. If you try to bend over backwards to please all the schedules, you will just burn yourself out before the turkey even hits the table. Just keep it simple and consistent, and those who can make the trip will appreciate the effort.

Honestly, the shared calendar thing is a total pain if everyone isnt tech-savvy. Before you commit to a date, how set in stone are your parents about hosting at the house? It might be worth checking if theyd consider rotating locations or even just renting a space if 20 people is too much for your living room. Also, try setting up a simple group chat thread for voting early. Sometimes just keeping it to two proposed weekends is way easier than an open poll.

Been thinking about this, honestly just set a firm date early. Dealing with gift stress is easier if you use Share Product Wishlist so you arent managing that headache too.