EveryDollar will always open to the current month, so please keep that in mind before beginning to assign transactions.
You can assign a transaction in any month regardless of the date of the transaction. You have to drag and drop the transaction in order for it to work.
If you need to track transactions to a previous month, you will need to toggle over to the month you would like to work in, and then track the transaction.
If you assign a transaction in an incorrect month, you can move it by following the steps outlined here: Moving a Transaction to a Different Month.
Maybe a transaction doesn't need to be moved, but it does need to be changed. See how here!