[Karinya]

I believe windsday or wind weather strengthens wind, and weakens earth (because wind is strong vs. earth). Weather has a stronger effect than day. Intense weather (two icons) has a stronger effect than normal weather.

Staves work the same - ice staff strengthens ice spells and weakens wind spells (which is why your third staff after ice/dark should probably be wind, it's the one element you really don't want to be using ice staff for).

As far as magic attack bonus... there are few ways to influence it other than job and level, but some of those ways are really powerful (e.g. moldavite earring).