The basis of element-day theory is that there are 8 days in the Vana'diel week. Each corresponds to one of the 8 elements. This is different from the real world where there are only seven days per week.
Firesday --> Earthsday --> Watersday --> Windsday --> Iceday --> Lightningday --> Lightsday --> Darksday
A good mnemonic for remembering the order is: FEW WILL Die. Also if you sort the Summoning Magic list after obtaining all Prime Avatars, it will sort by the order of the days.
- 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
The prevailing hypothesis: If you craft on the day of the crystal you're using (e.g.: using Fire Crystals on Firesday), you have a better chance of success, but a lower chance of skill ups and high qualities. Following that same logic, if you craft on the day that is strong against the crystal you're using (e.g.: using Fire Crystals on Watersday), you tend to have more failures, but more skill ups and high qualities. Lightsday is said to be just a good day overall to make sure you succeed - not good for getting skill or high qualities though. And Darksday would then be good for getting skill and high qualities, but more chance for failure.
Another idea specific to HQ items is that Lightsday may be good for success and bad for HQ for Light elements (Light, Lightning, Wind, Fire) and gives a higher HQ chance for Dark elements (Dark, Water, Earth, Ice). Therefore on Darksday you would have a higher chance of HQ for Light elements (Light Lightning Wind Fire).
These hypotheses are not proven and are points of contention among players who have a long record of several thousand synths. Many long-time crafters maintain that there is no direct correllation between days of the week and synthesis results.
Days and Spells
The days of the week have an influence on spell power. For instance, Lightsday occasionally increases light magic potency by 10% (Cure, Banish, etc) . Likewise, Darksday occasionally decreases Cure potency by 10%. All magic Astral Flow blood pacts are more powerful when used on the day corresponding to their element.
Fire melts Ice
Firesday: Weavers' Guild, Carpenters' Guild, San d'Orian Goblin Vendors
Earthsday: Tenshodo Guild
Watersday: Smithing Guild
Windsday: Boneworkers' Guild, Bastokan Goblin Vendors
Iceday: Goldsmithing Guild, Tanners' Guild
Lightningsday: Fishing Guild
Lightsday: Alchemist Guild, Windurstian Goblin Vendors
Darksday: Culinarian Guild