I don't know that you need to tell them today, but please tell them directly with enough time for people to make changes in arrangements if needed.
My mother was from a family of 9 children. For about 50 years the sisters each had their assigned holiday, and Christmas was always my mother's holiday. The other families had always had their individual celebrations on Christmas Eve. Each family had their traditional contributions for our Christmas Day dinner - we provided the turkey, gravy, a salad and some desserts. The same family brought mashed potatoes, dressing, sides and extra desserts, etc each year from the other families.
After my mother died I took over her hosting duties at her home (I lived out of state) for several years. But one year my cousin (who'd taken over for her mother for many years) decided not to come - but SHE DID NOT TELL ME. Now I (a single person) had hosted approximately 15 members of her family for years and years.
I do not blame her for wanting to change tradition, but it was cowardly and so inconsiderate for her to just not show up. Her brother called me at midnight to let me know (far too late to make any changes) as he was appalled that she had not told me that none of them were coming. We had no dressing, mashed potatoes or green beans the next day. Thank goodness her brother called me or I would have been waiting dinner on them.
Now that was cruddy (%#^&**^%#$) behavior. Ask me how much help I've been to her as she's dealt with breast cancer, death of her husband, and a heart attack. Now I have not done anything overtly mean to her. I've even delivered a casserole or two - but in terms of really being there for her - no.
When I decided (a couple of decades later after taking over for my mother) that I was going to opt out I made it clear to everyone months ahead of time that my DH and I were going to spend Christmas in New Orleans. For a couple of years earlier my sister had hosted when I was living in a tiny condo. But as my sister is a vegetarian and non cook I still had to provide the turkey, gravy, dressing, pies, homemade rolls, homemade cranberry salsa, sweet potatoes and a side vegetable. So much work and then I had to transport it.
For the past two years my DH and I have skipped out - New Orleans has been great. We have a condo there with a great little boutique hotel next door. We've eaten there and then waddled home.
But I have committed this year to host again - but I've decided not to do the whole big Christmas dinner thing. Most people have had multiple turkey dinners by Christmas day. I'm going to make jambalaya - and I do a bang up job of it since my NOLA friends have taught me well. I've started telling people what it's going to be.
Funny thing - my sister the 35 year vegetarian will have a piece of turkey on Christmas Day with cranberry sauce. She's the only one who has complained that I am not planning on doing a turkey.