Sealed Envelopes go on sale the first day of every month and are released on the 15th of each month. During August and September 2008, the envelopes were unsealed on the 8th, but on October 2, 2008 it was decided that the release date should be returned to the fifteenth. The Sealed Envelope disappears from the user's inventory and is replaced by a "Thank You Letter" for the current paid month (for example, a "Thank You Letter for August 2004" represents a payment made in that month of that year). Users are then able to open these virtual letters to choose one of two or three limited-edition items to be granted into their inventory from each letter. These items were known as Donation Items until potential legal problems prompted the name change. Monthly Collectible items and the "Thank You" letter items they are found in are specific to the month that they were released in: for example, the items found in the "Thank You" letter for April 2006 can only be found in that letter, and one could only purchase that letter in April 2006.