She suggests that the day this happens should be the first full moon of April, which this year falls on 4/20 (snicker), or this Sunday. Leiandra also says that it's fine to extend the "day" to the day before and the day after, in order to get all the guildleader appreciation possible in. So today's the day it starts -- make sure to thank your guildleader this weekend.
And we'll have to either include raidleaders, or come up with something else for them. Guildleaders do a lot of work, but raidleaders are out there in the trenches, too, making sure we all get the loot we need. If we're going to appreciate guildleaders (and we should) raidleaders need love, too.