The Harvest Moon Blast is only available during the Harvest Festival.
The blast can be made by mixing 20 Songpyeon, 20 Harmony Mooncakes, and 20 Longevity Mooncakes in any Chemical Combiner.
There is a slight chance for the blast to spawn trees bearing comet items. New autumn blocks and wallpapers released in 2015 were discovered using the blast.
Additional Info:
This item was released on 18th September 2013. It can be bought for 20,000 gems from the Harvest Pack. One pack gives you one blast. Blasting it creates a harvest moon-blasted world with a big moon background and random trees that were planted and fully grown. A Moon Mask (in 2013) and one of every mooncake (Longevity, Harmony, Peace, Prosperity), were also included. Hand Scythes were taken out of the pack in 2014, and was replaced with Balance Mooncakes.