What's with the yellow shirts?

I spent most of yesterday (Monday) wondering why so many people were wearing yellow t-shirts, most identical in colour and shape with the same symbol on the left. The first person I asked simply said, "it's our Monday", finding it difficult to explain any further in English. After a few more attempts I was told that it's because the King was born on a Monday, so every Monday the Thais celebrate the birth of the King by wearing these shirts.

Needless to say that the King is very popular here, you see it everywhere you go.When you go to the cinema, everyone stands before the movie starts for a video montage of the King, accompanied by what I think is the national anthem. Not too sure, does anyone know?