The drier months for trekking the inca trail are from April to November. On average these months have little rain making these months the best time of the year to go. The wettest months are December and January with on average over fifteen wet days each year.
The average maximum temperature changes little during the year. However due to 
the high altitude night time temperatures drop dramatically. In June July and August the temperature can drop to below freezing at night time.
Check the hiking essentials page for an overview of equipment you will need to take with you.