The date this card was released/announced.
In any case possible, this date represents the exact date a press release was issued announcing the card. Traditionally there is a one to two month lag time between announcement and retail availability, but this is becoming rare lately as manufacturers try to gain as much good press as possible by having cards on store shelves the day they are announced.
Some cards are never formally announced by the manufacturer and just quietly appear on retail websites. In this case, the release date reflects the first the card was seen in an official capacity on a manufacturer website or for sale in retail. If the date lists 00 for the day, that indicates we aren't sure of the exact day.
Some cards are announced, but never actually make it to retail availability. These cards are listed as "never released".
Sometimes we get reliable information about a cards specifications before the card is officially launched. In this case the release date will be marked in red until the date of the launch to indicate that the specs may not be final.