The shirt being referenced to can be found here, here, and here; the light blue shirt with the dark blue collar that has been used in multiple movies.
Harry has been noted wearing this shirt on an innumerable amount of occasions during the movies, and is almost never seen without it. Personally, I've seen Harry with this shirt primarily within action scenes (such as the Forbidden Forst in HP:DH, when Harry hears someone in his Uncle's house, in OOTP etc.). Does this pattern have any relevance with the action scenes at hand?
I am mainly looking for a history on why Harry is countlessly seen wearing this shirt, and why? Where does it come from (in-universe), and who gave it to him?