This Limited Edition item was available for 350 MC from October 30-November 21, 2010 and gave 1 xp when bought. From December 29, 2010-March 3, 2011 it was available by purchasing 7200 MCs($59.99 USD) or 12500 MCs($99.99 USD) from the offers page. The description reads "Contains 1 of 5 exclusive seasonal surprises!" True to its description, all items inside the Harvest Mystery Crate cannot be bought elsewhere. What you get from the Harvest Mystery Crate is entirely random however, so there is no way to pick and choose, or to "increase" your chances. It is possible to receive the same item more than once if you purchase the crate more than once.
This crate is not the same object as the Mystery Crate or the Mystery Gift Box, nor will it give the same items as the other two.