People are confused as to how to sit shiva and how to properly pay a shiva call. Because people do not know, and because talking about death makes people nervous and awkward, the shiva house often turns into a festive gathering filled with nervous chatter, instead of the proper house of mourning.
The average number of days in a month is around 30.4. With 24 hours in a day, this is an average of about 730 hours per month.
So on average, the combination of any 3 days occuring 5 times in a calendar month will happen once a year. In any given year, there are only 6 such combinations of days. This is because 2 months with 31 days start on the same weekday.