If you plan to import historic giving records from another system, it is best to complete the import before enabling the first gift triggered communication series. The series will fire on the first gift received from an individual regardless of how that gift comes into Vision2 as long as:
The triggered communication series is enabled for your organization (all designations) or for specific designations
The gift date is not earlier than 2 weeks before today
The emails will be sent based on the scheduled campaign series you configured for first gift.