"Er isst am Mittag."
Why do you say "am Mittag" instead of "an Mittag"? The literal translation is "He eats at the noon." Why does this noun require a "the"?
It's just a set rule. You always say "am + Tag" and "im + Monat" and "im + Sommer/Winter/etc". It doesn't sound right in English, but that's just the rule.
But isn't that because all names of days, months and seasons have male sex so it is short term of an dem?
So it's supposed to be translated "midday" not "mid day".. I guess I have to practice my English as well.