Probably not. We compared the attendance numbers and weather for K-Days from 1994 to 2014. There is not sufficient evidence to suggest that rain or rainy days has a major effect on the number of people coming out to K-Days.
The most poorly attended years for the festival were in 1995, 1996 and 2006. Those years brought in 677,143, 683,081, and 688,369 revellers respectively. However they were the 3rd, 13th and 14th rainiest years. The rainiest year, 2014, had 740,840 enter the gates and sits in the middle as the 11th highest attended over the past 21 years.
The best attended K-Day was in 2005, with 810,503 visitors. It was also the 6th most rainy year.
Below is a slope graph. The left shows total millimetres of rainfall during the festival, the right show total attendance. Follow the lines to compare overall rainfall against attendance.
So what's happening? This data looks at total rain by day only. If the rain were to fall during the off hours (overnight) during the 10-day exhibition, the weather would likely have a smaller effect than if it were to rain during the day. Hourly precipitation data was not available for comparison.
Another caveat is that a lot of rain may fall in a short period of time. For example, in 2014, 69mm of rain fell on July 25th alone. That's 86.25% of all the rain during that 10-day span.
Our calendar below shows the distribution of rain over the ten days. The darker the square, the more rain fell on that day. Hover over a square for the rain total for that day.
Consider 1995. It was the least attended year, but not the rainiest. However, it did have more than 2mm of rain on six of the 10 days and at least trace amounts of rain on nine of the 10 days. 2001 is similar with eight of the 10 days with at least trace amounts of rain and 61.2 mm total.
The major outlier in all this data is 2005. The best attended year for K-Days was the 6th rainiest and had rain fall eight of the 10 days.
What made 2005 so popular?
Rick Carter, Northlands director in charge of K-Days, told the Journal in 2005 that "If you invest in better acts and entertainment, people will come."
That year featured Matthew Good, Jeff Healey, Finger 11, Carroll Baker, and Twisted Sister. The heavy metal icons brought in a record attendance of 103,733 on the final Friday night.
So maybe it's as simple as that. Bring in the acts the people want and they'll come out, rain or shine.