In this example, it depends on when the check arrives in your mailbox. If it arrives between 12/28-12/30, it is 2023 income. If it arrives in the mailbox on 1/2/24 or after, then it is 2024 income to you (even if it is a 2023 expense for the payer).
Can you ask the payer to put the check in the mail on Friday the 29th? That would probably prevent you from receiving it in 2023.
Also note, if the check is post-dated (for example, the check is dated 1/2/24) then it is 2024 income to you no matter when you receive it, but it is also a 2024 expense for the person who sent it (because it can't legally be deposited until 2024, so you haven't received it until it's valid, but the payer hasn't spent it until it's valid.)
