"And before the dinner?"
Translation:Et avant le dîner ?
Le dîner can mean that dinner right there that we know about.
Le dîner can also mean the idea of dinner, dinner in general
In theory, Et avant le dîner ? can mean either that particular dinner or every dinner. However, there are lots of exceptions to rules in French, especially about common practices like eating etc. and this may be one of them.
I can see why you wanted to use du dîner as you wanted to expand beyond the particular dinner that we all know about. The problem is that usually du = some, more than one but less than all. Therefore your usage will not encompass all dinners as you intended.
Hope this helps.
'De le' is contracted to 'du', so 'de le' would be wrong in any instance. I'm not sure why, however, 'du diner' would be incorrect, except, perhaps, 'du diner' would indicate a special event. northernguy expounds on it a little bit (above) but doesn't give a definitive answer.
You employ "avant que" only when you follow up with a verb. Here, in this sentence, we just have a noun that follows "avant" so no "que" is needed.
An example for "avant que" would be:
Cache le cadeau avant qu'il ne le voie. Hide the gift before he sees it.
Note that you have to use the subjunctive then!