Days of the Week

There are 8 days in the Vana'diel week. Each corresponds to one of the 8 elements.

Here they are in order:

Firesday --> Earthsday --> Watersday --> Windsday --> Iceday --> Lightningday --> Lightsday --> Darksday

Rinse, repeat.

Days and Crafting
There are many people who feel that the day of the week strongly affects the sucesses and high qualities of your crafting.

Basically, if you craft on the day of the crystal you're using (ie: using Fire Crystals on Firesday), you have a better chance of success, but a lower chance of skill ups and high qualities.

If you craft on the day thats strong to the crystal you're using (ie: using Fire Crystals on Watersday), you tend to have more failures, but more skill ups and high qualities.

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.