I have this exact problem. I was on an Apple Support Chat about to set up my phone as new when I decided to Google "iphone activity app crashes when i view a day" and here I am. Thank goodness I'm not alone. I'm going to sit tight until the next iOS update and cross my fingers. Meanwhile if anyone finds a solution, let me know. I've got an iPhone 5S running iOS 9.1 (13B143)
Things I tried that didn't work...
1. Power off and on, normally.
2. Force a reset, holding power and home buttons.
3. Settings > General > Reset > Reset All Settings.
I can view the month of November, when I either tap on November 1 or swipe to it when viewing day by day, the day and data appear, the rings animate around. As soon as the animation finishes, crash! Every time. At this point, I am due the Achievement for completing all rings for every day in November.