Why is el día masculine?
I wonder whether there is a reason that "día" is masculine. With "agua" it's clear — it starts with an "a" and saying "la agua" is awkward. "Foto" is also understandable — short for "fotografia".
5 Answers
The Latin word for day was "dies", from the 5th declension, and it was used mainly as a masculine word (sometimes also as feminine), and this gender was kept even though the word change to "dia" in Vulgar Latin. In Medieval Spanish, "día" was the only masculine word in -a, and "mano" the only feminine in -o, but nowadays there are many more words that don't follow the pattern.
It's just an exception: This link may help 
Four of the nouns that end in -a are simply exceptions and must be memorized.
el día el mapa el planeta el sofá
Well, there has to be a reason. Real Academia Española could not have just decided: "no hay suficientes excepciones en Español".
I checked the etymology and found that "día" comes from Latin "dies", which happens to be masculine. I suppose that it was a common enough word, that the gender remained, even though the ending changed. And the ending must have changed, because that's how it sounds.
On the other hand, for instance, "noche" comes from "noctis", which is feminine, as we expect it to be.
While we're on that subject, it's one of Top Ten Common Mistakes When Learning Spanish
Edit:
Check out this other great link: Gender of Spanish nouns
Yes. The Spanish language is often so logical that I get lazy and forget to memorize the exceptions . 
