Use the valid dates when the date is only valid for a specific period of time. For example, a constituent is moving abroad for one year (2005). When finished, he will move back to his current address. The current address would have no valid dates and would be the preferred. The abroad address would have valid dates of 1/1/2005 to 12/31/2005. Both addresses should have the Send mail to this address checkbox marked (if you will be sending mail there).
Use the seasonal dates when the address is not valid for the entire year. For example, a constituent is attending college. He lives at the college during the school year (August through May), but returns to his home address during the summer. In the Seasonal Dates, enter 8/1 to 5/15 (e.g. the dates of the school year). The home address has no seasonal dates and is the preferred. Both addresses should have the Send mail to this address checkbox marked (if you will be sending mail there).
When running a mailing, mark the Consider addresses with seasonal dates checkbox on the Ind. Address tab.