Some friends and I are planning to hike the Inca trail sometime this year. Another friend who has done it told us July isn't the best time because it is very crowded and daylight hours are short, making the hike more rushed. On the other hand, we don't want to go in the middle of rainy. As a result, we're thinking we should go either in May or September. Does anyone have any thoughts on which time would be better? Could we even pull off going this May if we haven't started planning yet? I understand Inca trail trips fill up quickly.

May is almost certainly filled up now. The 4-day trek is usually booked up 3-6 months in advance.

You can see the number of available spaces for the 4-day trek at

http://www.inca-trail.com.pe/

OK, but what about September? Is that good time to go?

High season is May to August so yes. September is before the rainy season.