Complex sentence: Starting in December, they will have completed the courses in the end of May, and first then, the final group is picked (6 men or sometimes less), just 2-3 weeks before going to Greenland for 26 months in a row.
Split into simpler sentences: Starting in December, they will have completed the courses by the end of May. The final group is picked (6 men or sometimes less), just 2-3 weeks before going to Greenland for 26 months in a row.