How to account for sponsorship invoices sent in the fiscal year prior to the event?

Revenue is indeed allocated to a given month based on the invoice date. You will need to make use of a JE to deferred revenue in Dec if you don't wish to recognize it yet. Then move it out of deferred in the next period. 

Thank you. I was looking into deferred revenue and found that that was the right option after posting. However one thing I don't get is how to deal with this revenue if we sent an invoice to a sponsor before the month of the event but they paid after the month of the event (so date of invoice, date revenue is earned, and date invoice is paid are each in different accounting periods). How would I account for that, as the revenue would be unearned on the date of the invoice but accrued on the date of payment? If it helps, I currently have the deferred revenue sponsorships set up by tying the "Sponsorship" service billed on their invoices to the deferred revenue account, rather than making a JE every time. 

So in December, you have an entry to deferred revenue to account for invoices booked before the event, in January it is reversed to recognize that revenue in Jan. Invoices dated in Jan are all good because they're in the month revenue is incurred. As far as revenue goes you're all set. The unreceived payments for any invoice are reflected by a balance in Accounts Receivable. Basically you've earned the revenue, it's booked, and you have the AR account showing money is owed to you and waiting to be received. This is the function of the AR account, to act as a pending asset until the cash is received and becomes a more tangible asset. The revenue discussion is over and now you're only concern is the flow of cash.