'Da', in street language, is the principle definite article and equivalent to 'the' in more formal English.
The use of the the /d/ derives from two main aspects:
-quicker and easier to ignore the 'th' sound, usually for non-native English speakers (such phenomena is prominent in the word 'ting' rather than 'thing.')
Dumbledore's Army. Started by Harry Potter, Ron Weasley and Hermione Granger in 1995 in their 5ft year at Hogwarts. Was later run by Ginny Weasley, Luna Lovegood and Neville longbottom in 1997.
Started Because the school was no longer teaching them how to protect themselves against the Dark Arts.
Headquarters: Room of Requirements
Harry, Ron and Hermione started DA becasue Umbridge was only teaching theroy in Defence against the dark arts.