"What happens on a normal day?"
Translation:Was passiert an einem normalen Tag?
Prepositions are pretty much always idiomatic and the idiomatic preposition for days in German is ‘an’, so ‘auf einem’ isn't correct.
That would be “on the normal day” (“am” is the contracted form of “an dem” = “on the”).
Does mixed inflection like in normalen always take "en" at the end of the word?
I don't like the terms "strong/weak/mixed inflection", because they just lead to creating three categories each with some combination of rules and exceptions. Adjectives in mixed inflection do not always end in -en.
I find it better to understand the logic behind the endings. That way I can tell you something like "einem already shows the gender/case signal, but it's a modified signal, so the adjective ending must be -en". Such reasoning is mentally quicker to look up than the combination of tables from the "strong/weak/mixed" way of looking at things.
Any time you change the article (in this case the dative requires changing from der Tag to dem Tag), the adjective changes to an -en ending. In plural, it always ends with an -en no matter what.
"on a day" is metaphorically considered a location, so an takes the dative case.