I just dont get the difference between de...à ..and.de..du...they e both used in between dates?

Bonjour Jayne !

The difference here is between "de...à" (from...to) and the contracted forms "du...au" (from *the*...to *the*).

So with days of the week for example, you use:
- "de...à" for specific days (from Monday to Sunday), because to say "on Monday" in French, you don't use the definite article ("Lundi, je suis allé au marché.").
- "du...au" for general days (from Mondays to Sundays), because you use the definite article when generalising in French ("Le lundi, je vais à la piscine.").

Same issue with dates: you use "du...au" because to express dates in French, you always use the definite article ("le 16 juin 2016").

I hope that's helpful!
À bientôt !