To say "every evening", they say "jeden Abend". Abend is male. So I would have said "jeder Abend". "jeden" sounds like of accusative form. Why is it jeden?
Add : I mistakenly said jeden is dative, but corrected it to be accusative.
Time specifications frequently require the accusative, especially if coming without a preposition. See here for more examples. Also there is no dative to recognise in your example.
The behaviour of jeder is well discribed here. If you don't understand everything there just look at the first table.
According to that, jeden is the accusative form of jeder, not the dative.
So you can't decide in general if it is jeden Abend or jeder Abend, it depends on the function of Abend in this sentence: is it subject, dative object, accusative object?
Here are some examples:
Every evening is the same.
Jeder Abend ist gleich.
Every evening we do the same.
Jeden Abend machen wir das Gleiche.
An jedem Abend machen wir das Gleiche.
To learn more about this topic you might look into general German grammar. For a first glance you might take this and replace every article with the respective form of jeder. This works perfectly fine, even if some sentences don't really make sense anymore ;)
To take off on Roy's great answer, the more common constructions use either the accusative or dative cases.
During Every evening we do the same." Here, there is an implied word, during. That's why "jed--" is not in the nominative case in German. Instead, there are two ways to render it.
An jedem Abend machen wir das Gleiche. We're using a preposition, "an," which makes jed--, jedem, putting it in the dative.
Jeden Abend machen wir das Gleiche. Here, jed-- becomes jeden, in the accusative, because there is no preposition. This is an "adverbial" accusative.
Then there is the nominative example, Jeder Abend ist gleich. Every evening is the same. (There is no implication of "during.") So "Jeder Abend" remains in its "original" (nominative) form.