The trade confirmation or brokerage statement from when you purchased the stock will show the purchase date. If you no longer have the confirmation or statement, the brokerage firm that handled the purchase for you should be able to tell you the date.

If you have no record of the date, and the brokerage does not have a record of it, you will have to use an estimated date. Make your best guess of when you bought the stock. The exact date is not really important, as long as you correctly identify the length of time that you held the stock as short-term (one year or less) or long-term (more than one year). You can also enter "Various" as the purchase date, but again you have to correctly identify it as short-term or long-term.