The behaviour of *jeder* is well discribed [here][1]. 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.<br>
Jeder Abend ist gleich.

>Every evening we do the same. <br>
Jeden Abend machen wir das Gleiche.<br>
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][2] and replace every article with the respective form of *jeder*. This works perfectly fine, even if some sentences don't really make sense anymore ;)


  [1]: http://www.canoo.net/services/OnlineGrammar/InflectionRules/FRegeln-P/Pron-Indef/Pron-jeder3.html
  [2]: http://www.nthuleen.com/teach/grammar/nomakkdatexpl.html