Gotobi is the term they use in Japan to referring date of multiples of 5, example 5, 10, 15,20,25, and 30. Those are the busiest days for business. These days are apparently ordained by accountants.
Every company are doing auditing, collecting money and things like that. Payment in business is usually settled on these days in Japan and it causes heavy traffic jams.
It cause many crowd and makes traffic. It’s just one of the theory to explain traffic jams in Japan.