There are 8 days in the Vana'diel week. Each corresponds to one of the 8 elements.
Here they are in order:
Firesday --> Earth --> Water Crystal --> Windsday --> Ice --> Lightning --> Lightsday --> Dark
Rinse, repeat. A good acronym is FEW WILL Die.
- Each day lasts for 24 game hours from 0:00 - 24:00 (57 minutes, 36 seconds Earth Time).
- There are 25 Vana'diel days per Earth day, 175 Vana'diel days per Earth week (according to above).
Days and Crafting
Lightsday is just a good day to make sure you succeed - not good for getting skill or high qualities though.
And Darksday is good for getting skill and high qualities, but more chance for failure.
This, of course, is very flexible and it seems to have a stronger effect when you're in your higher levels. You can craft for skill on the day of the crystal you're using. You can get high qualities on the day of the crystal. You can get skill on Lightsday, etc.
Days and Spells
The days of the week seem to have an influence on spell power. For instance, Lightsday occasionally increases cure potency by 10% when cure spells are cast. Likewise, Darksday occasionally decreases Cure potency by 10%. Carbuncle's Searing Light and other Astral Flow blood pacts are more powerful when used on their corresponding day.
Fire melts Ice
Light illuminates Dark
Firesday: Weavers' Guild, Carpenters' Guild
Earthsday: Tenshodo Guild
Watersday: Smithing Guild
Windsday: Boneworkers' Guild
Iceday: Goldsmithing Guild, Tanners' Guild
Lightningsday: Fishing Guild
Lightsday: Alchemist Guild
Darksday: Culinarian Guild