<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by ExMo:
MHONE and MLONE refer to H[igh] and L[ow] season travel. High season is generally June-August and most of December. Low season is the rest of the year. The determination is made based on the starting date of your trip, not the duration of it.</font>
I think that the rules are even more specific and state that the date of the first intercontinental flight is used to determine what season the ticket should be priced on.