I would not 'forget' Thanksgiving as revenge for their forgetting your birthday. That would be petty and not address the real issues.
Is there some basic problem in your relationships? Could they have been doing what you are now threatening to do, and neglecting your birthday as a 'punishment' for some real or imagined neglect on your own part? If you are sure that this is not the case, it seems to me that there are two main possibilities: (1) bad communication, combined with different attitudes to birthdays in general or lockdown birthdays in particular; or (2) everyone has got into a habit of regarding you as the person responsible for organizing all holidays and celebrations.
(1) Birthdays are much more important to some people than to others. I am at one extreme: I can't bear to have my birthday acknowledged at all, and if people directly remarked on an age milestone ('Happy 50th!') they'd be lucky if I didn't boil them in oil! Fortunately my family and close friends know this and don't remind me of my birthday; and other people don't know when it is. I know I am extreme, but not everyone likes to be reminded of age-milestones. And some other people love their birthdays but hate surprises and like to be in control of what happens on their birthday, so would expect others to follow their directions rather than do something spontaneously.
There's also another issue this year. I notice increasingly that some people regard birthdays and other special occasions mainly as 'fun' social events, and therefore not having much point if lockdown and social distancing rules mean that such events have to be cancelled or drastically scaled down. For others they are mainly family occasions, and may become even more important when social life in general is restricted. I am guessing that perhaps most of your family are of the first type and you are of the second. Do you usually make a 'thing' of your birthday, or has it become more important due to lockdown? I'm asking because if you have a big birthday celebration every year, it's a little surprising that ALL your family would suddenly forget. (This is all assuming that you are indeed under pandemic-related restrictions. As you mention Thanksgiving, I assume that you are American, and realize that in that case, such things would be decided on a state or local level, rather than a national level.)
In any case, I would address it directly, rather than retaliate by 'forgetting' Thanksgiving or Christmas. Say something like 'Birthdays may not be important to you, but they are to me; and I feel quite hurt when you ignore mine.' If they don't respond, then I think you have bigger problems to address in your family relationships than one neglected birthday.
(2) They may have all come to see you as the 'holiday organizer', and maybe you see yourself that way too. Perhaps you could say, with regard to Thanksgiving and Christmas, that you look forward to these festivals, but find it increasingly difficult to take all the responsibility for all the organization, and will need help from them if these are to take place this year. If they aren't keen enough to put some of the work in, or don't think it's worth it at a time when such festivals must be scaled down, then there can't be a big 'do' this year,