Restricted Stock Units, delivered , but can't be sold until 2022

I have slightly unusual rules covering a Restricted Stock Award from my company. The company set up a schedule where  stock is "delivered" yearly on a specific date, but the stock is still restricted such that it cannot be sold until the final delivery (in 2022). If I left the company now, I would still own the stock delivered up to today, but I still cannot sell any of that stock until what would have been the date of the final delivery in 2022.

So when is the 'taxable' event? When I can actually sell any of stock (in 2022), or is it/was it when the stock was 'delivered' to me.