Complex sentence: The pass is easier to cross earlier in the season and can be attempted as early as the last three or four days in June; earlier and one is likely to be the first party of the season to break the trail.
Split into simpler sentences: The pass is easier to cross earlier in the season and can be attempted as early as the last three or four days in June. Any earlier and you are likely to be the first party of the season to break the trail.