Is your site more pleasant in winter or summer? For example, Sweden would only work in the summer; a tropical site would probably work best in the winter. Do you have other obligations during either term? Think about research, teaching, conferences, child care, family events. When can your host(s) best accommodate the group? Maybe they have other groups coming in January or June, or the locals are away on holiday during all or part of the month. Keep in mind that it is generally more difficult to recruit for summer than winter. We typically run about 12 summer programs versus over 35 winter programs, yet more summer programs are canceled each year than winter programs.