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.
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.
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