What is the significance of the 3rd, 7th and 30th day after Catholic funerals?

I was reading a 1952 missal and wondering about some of the customs, one notable one I saw was that there was a particular commemoration that is supposed to be celebrated on the 3rd, 7th and 30th day after a funeral. Is there any particular significance in these days in Scripture or Tradition? How would someone go about getting a Mass said like this? Assuming there are funerals every other week in an active parish, wouldn't every other Mass wind up being this kind of commemoration?