I myself did the Shikoku Henro Pilgrimage from the end of January to the end of February using 35 days. I personally think this was the right choice because of following reasons.

• There’re not so many pilgrims, so it was fairly easy to find accommodation and get “goshuin” (temple stamp) on the “nokyo-cho”
• I enjoyed colder weather (around 10 degrees) while I was walking more than 30 km/day every day

However, I would say that the best season to do Shikoku Henro Pilgrimage is Spring or Autumn mainly because of following reasons.

• It’s not too hot nor too cold
• The days are getting longer
• There’re not so many insects nor snakes
• In Spring, you’ll see a lot of flowers blooming (I walked through several nice places where Sakura (cherry blossoms) would be beautiful)

On the other hand, there’re more pilgrims during these seasons, hence you might face following difficulties

• It might be difficult to find accommodation
• It might take more time to get “goshuin”

In the end, it’s still up to you, because you can divide this Pilgrimage trail in several occasion, so you may come to Shikoku in different seasons. You may also use train or bus.