Initial testing suggests this has the same behavior as Sorcerer's Tonbans, providing a 3% boost in damage if the day matches the spell's element (eg. Thunder IV on Thundersday).
Obtained from the completion of the Windurst Waters [S] quest "At Journey's End". Need to be on Mission 38 of Wings of the Goddess "Adieu, Lilisette" and completed the previous quest "Manifest Destiny".
@Totema : I can see how you might have gotten this idea from the wiki talkpages, but if you check the data you find that the ring does *not* nullify penalties from casting on an element's opposed day. It simply doesn't "enhance" the penalty, so its the same as not wearing it on those days.
If you're getting this ring purely for COR, you won't see much of a difference in damage versus a MAB+3 Ring from Abyssea - Tahrongi. For Corsair, MAB works like a percentage boost to the base damage of quick draw, then after you factor in MAB, you factor in elemental staves and weather/day gear after the staves. In other words, unless you're using the staves (Which I hope you are) you won't see a boost in damage versus the MAB+3 ring.
Example:
(Base Damage)*(MAB)*(Staff)*(Day Bonus)
With Zodiac Ring
(1)*(1.10)*(1.15)*(1.03)
1.1845x (10 being your MAB, overall MAB/Base damag will vary)
1.30295
With MAB+3 Ring
(1)*(1.13)*(1.15)
1.1845x (13 being your MAB, overall MAB/Base damag will vary)
1.2995
With actual QD numbers:
Zodiac: 502 (Without Obi)
MAB+3: 496 (Without Obi)
Also has the effect of nullifying any spell power penalty due to casting on a day that opposes the element of your spell. So Fire IV cast on Watersday would be just as powerful as if it were cast on Earthsday.