The Japanese had used complex lunar calender until 19th century. A month had 28 days then. A month also had 5 weeks, and a week had 6 days. The six days included Taian, Butsumetsu, Senpu, Tomobiki, Shakko, and Sensho. They are called Rokuyo.
According to the old lunar calendar, there the day which comes around on the fourth of every sixth day is called Tomobiki (or "taking along friends"). Few if any funerals are held on this particular day in Japan, for a funeral service observed on the Tomobiki-day will soon be followed by another funeral in the same family (bringing friends along as the name of the day implies). If a funeral must be observed on this day, a doll is put in the coffin, so that it will take the place of the one whom death would otherwise claim.
Tomobiki itself is not considered an unlucky day, but it can be. If you have good fortune, your friends may have good fortune too. But if you are one who brings his bad luck along with you, on this day it is thought that the very same bad luck will visit with the people around you.
Relevance to Urusei Yatsura (and this site)
"Aha!" I hear you say. It's starting to all make sense isn't it?
The name of the township where Urusei Yatsura takes place is of course called Tomobiki-cho (Tomobiki town) which is a name directly linked to the name of this calendar day. It's no coincidence then that the superstitions that are believed for the day of Tomobiki seem to apply to the fictional town as well. Invariably every time strange happenings occur in the series, it's never an isolated incident. If one character gets caught up in some misfortune, you can bet that soon many more cast members will be dragged along for the ride.