I created an event on my Mac's calendar that was scheduled to repeat every Tuesday and Wednesday. Later, on my iPhone, I wanted to change the repeat interval to Wednesday and Friday instead, but I couldn't figure out how to do so.
When I went to edit the repeat interval for the event on iOS, the only options were (a) the custom interval for Tuesday and Wednesday (selected), (b) none, or (c) the default intervals like daily, weekly, etc.
I thought that tapping on the custom interval would let me edit it, but instead it just re-selected it. However, when I selected a different repeat interval instead, the custom interval disappeared entirely, and there was no option to create a new custom interval.
(I also created a new event entirely and found that the Repeat settings don't include a "custom" option in that case, either).
So it is just not possible to create or edit custom repeat intervals on iOS? I'm running the latest iOS 7.1 on an iPhone 4.