There's a thing referred to as "constructive receipt". If you receive the check on or before Dec 31 of the tax year, then it's reported as income for that tax year. Doesn't matter when you actually cash it or deposit it to your business account.
I've a pretty good idea why you're inquiring about it, as it's more to keep the book keeping simple for each tax year, than it is a tax issue. I generally don't issue any invoices for my services after Dec 15th, and will wait until the next year to invoice my clients. I don't like billing someone in one tax year, and then recording the payment in the next year. I prefer to close out my books for a tax year with everything balanced and no carry overs.
