When Morrison Lodge was Chartered in 1875 there were not any street lights to aid in their navigation home. So like most Lodges of the time they held their meetings in conjunction with the full moon. This would give the most light to aid in their travels.
Since this time most Lodges have changed their bylaws to meet on a set night, "the third Monday of each month" or something along these lines. It makes it easier for the modern man to plan his future schedule. It really is not that hard to look at when the full moon is but it is an extra step that many men don't like doing.
Morrison Lodge has stuck with their traditions and still meets "the Monday on or before the full moon." If the full moon falls on a Monday we will meet on that evening. If it falls on any other day, we back up to the Monday just before it and we meet on that evening.