Apparently the Celestial Weapon crests and sigils in Final Fantasy X were named after days of the week in the original Japanese version. This seems quite likely to me, because as it turns out, they still are - it's just a bit less obvious.

First, the obvious one: Tidus has the Sun Crest, and the Sun's day is - well, Sunday. Yuna's Moon Crest is just as easy to figure out: the Moon's day is Monday. The rest are a bit more obscure, though. (Well, except for Kimarhi's Saturn Crest. Saturn's day? Saturday.)

Auron has the Mars Crest. Which day is Mars's Day? Why, Tuesday, of course. How is it Tuesday? Here we have to look back at the origin of our days of the week. They originally came from the Roman names of the week (which is why they're named after Roman gods). Tuesday was dies Marti, Mars's Day. (Which is why Tuesday in French is "mardi.")

How does dies Marti become Tuesday? Well, the English day names trace themselves back to the Norse gods. Mars became Tyr, the Norse God of Battle. Tyr's day is therefore "Tuesday."

So, without further ado, here's the complete list:

So there you go, from day of the week, to the celestial bodies that name the crest. Note that I'm fairly sure that the days listed here do not match the original weekday used in the original Japanese. They've been rearranged to more closely match the characters.

Note: You'll notice that Sunday and Monday don't have gods associated with them. You might be thinking "well, why not Apollo and Diana?" It's because the original Latin is dies solis and lunae dies, so the days are named after the actual celestial bodies and not the associated god.