why "debut juillet"

Question

Why just "debut juillet" instead "au debut de juillet"?  Apparently I can't submit this question without more detail.  I don't know how much detail I can give.  It's a simple question

Maarten Kamp · Accepted Answer

Carl, 
In the exercise, there are multiple possible responses noted, and other combinations are also possible.
Examples with ‘ au début de juillet ‘ and ‘ début juillet ‘ are given. Both are in usage, but in everyday speech it is quite colloquial to just say ‘ début juillet (mois/année etc) ‘. 
See Larousse (under ‘Expressions’) and wordreference links below :
 https://www.larousse.fr/dictionnaires/francais/d%C3%A9but/21948
 
 https://www.wordreference.com/fren/d%C3%A9but

Cécile Janty-Davies · Answer

Hi Carl,
Just to add to what Maarten has said, please take a look at my answer to a similar question -
fin janvier
Hope it helps!