All Tiller spreadsheet Transactions sheet should have the "Month" column. It might be a hidden column out to the right in the Transactions sheet. Tiller populates the month data for each transaction that it pulls in automatically.
If you're preparing historical data, or data for accounts Tiller can't automate, to manually add it to your Tiller spreadsheet you can use this arrayformula to have it automatically fill in the month data for the data set.
This assumes the "Date" is in column A. Change the formula to match the location of the date data in your prep sheet.