From: www.guildwars.com
In the first scheduled update of every month, one new flavor of the ever-popular transmogrifier tonics will be added to the Zaishen Chest, replacing the tonic from the month before. These special tonics, usable only in outposts and towns, will transform your character into the embodiment of that particular potion.
Open the chest for a chance to find one of these special tonics. If you're very lucky, you may even find an Everlasting tonic. These super rare variations have infinite uses, so they'll never run out.
Bookmark this calendar and be sure to check it in the beginning of every month to learn what new tonic has been added to the chest. Is there a tonic from a previous month that you just can't live without? Don't worry! The tonic schedule will repeat each year, so if you've missed one of your favorites, simply wait for the Zaishen Chest to carry it again.