FIS Rules:

http://www.fis-ski.com/uk/rulesandpublications/cross-country/rules.html

USSA Competition Guide:

http://ussa.org/PublishingFolder/default_1630.htm

Regarding race-day cancellation because of weather, FIS and USSA rules state:

The Jury must clarify and decide whether a competition shall be postponed, interrupted or cancelled. If
the temperature is below -20 ° C, measured at the coldest point of the course, a competition will be postponed or cancelled by the Jury. With difficult weather conditions (e.g., strong wind, high air humidity, heavy snowfall, or high temperature) the Jury may, in consultation with the Team Captains of the participating teams and the doctor responsible for the competition, postpone or cancel the competition