It's the RSPB's Big Garden Birdwatch in the last weekend of this month. People can sign up here. Signing up also gives you 20% off and free delivery in their shop so you can stock up on bird food and feeders.
The full results for last year can be found here (Excel file) in case anyone wants to have a play. Unfortunately they don't seem to show any comparison with previous years so it's hard to know whether species numbers are changing or remaining stable.
Big Garden Birdwatch 2020
Big Garden Birdwatch 2020
it's okay to say "I don't know"