I'm glad you know about the plan for a party.
Nothing worse than being dragged out of bed, in need of a hair wash, putting on your gardening clothes as first at hand, then being driven to a venue, to your worst nightmare of lots of people in a small room, all dressed nicely, singing "happy birthday". Especially as you had given strict instructions to never spring a surprise as you don't like and can't cope with surprises. Even more especially that several of the guests had also said the same to the party arranger. Needless to say, he was very soon after the ex.
That said, as you know about it, can you help organise it so that you can enjoy the occasion. You never know, they might club together to give you a little holiday as a gift.