While we can schedule these emails to send on future dates, the mass email will send to the accounts in the query at the time the email is scheduled. For example, if the current date is August 31 and we create a query to find all donations made Yesterday, the query will return donations from August 30. If we schedule a mass email to run each day for the next month based on this query, the email will always send to the people who donated on August 30 because it looks at the results of the query at the time the email is scheduled.