boost::serialization::save — Function to save last_day_of_the_week_in_month objects using serialization lib.
// In header: <boost/date_time/gregorian/greg_serialize.hpp> template<typename Archive> ( ar, lkd, );
last_day_of_the_week_in_month objects are broken down into 2 parts for serialization: the day of the week, and the month