The Acquisition Date is the date you acquired the item (which may be different from the actual purchase date). This may be today, a year ago, or could even be one month from now. The Acquisition Date should be the date the Item is available to start renting (when it has arrived in your warehouse, is in rentable condition, and is ready to make its debut at events).
Be as accurate as possible but, if you are entering a lot of Items at once as you're getting your RW Elephant account set-up, be sure that the Acquisition Dates you use are BEFORE any Orders you're planning to enter. For instance, if you're planning to enter all Orders you've had since the beginning of this year, you'll want to make sure that the Acquisition Dates for the Items are December 31st of the prior year or earlier.
If you plan to import your inventory (instead of entering it manually), make sure that this column is labeled "Acquisition Date" on your spreadsheet.