Down payment from client counts to what tax year

Will a deposit by a client in 2020 for services I will do exclusively in 2021 count to income tax 2020 or 2021? Is there a difference if they pay now via venmo or a check I can cash in 2021?

 

We're signing contract now, usually with a deposit right away but I need the deposit to count to 2021. I will be working with them  starting in 2021 for a few weeks and the deposit applies to the last two weeks of service.