So, if we were reading this strictly chronologically, the passover celebration would have come before the census in chapter one. Interestingly, it is only their second (the first one celebrated while still in Egypt) and it will be their last until they arrive in the Promised Land. I like that even foreigners can participate (however, they have to be circumcised first, Ex. 12:48).
That the bones of the lamb are not broken is said to prefigure Christ not having any bones broken.
King David is still following the rules about the trumpets when he is king, but he expands it to the whole orchestra when worshipping during feasts.