Places and months I've backpacked:
Alaska: March, June, July, August, September, October
California: January, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec between coastal, Sierra, Big Sur, and desert trips.
Washington: April through October
Hawaii: Jan, Feb, March, April.
New Zealand: March
Costa Rica: December
So, yeah, year-round. Year-round was easiest in California with its varied terrain and climates. Now, year-round trips involve more air travel (or snow camping).