It is a day where the Moogle shop becomes 20% cheaper on all items and stocks some rare items to boot. If I were you, I'd only do my shopping on the Moogle days unless you need something and can't risk dropping (like for a special portal or something along those lines). If you can spare a few drops to get the day, I'd recommend continually dropping until you see it in one of the worlds (drop from the world map) because when you buy a lot of stuff it can be quite pricey, and 20% off is a nice bonus.