Now, why would someone divide up the spring months into eight alternating half and full decades? Some method of calculating Easter? I don't think so, because Easter can never fall into May... Perhaps, there is an agricultural reason - four half decades represent planting dates? Oh well...
Two more sites on Gregorian / Julian calendars: http://europeanhistory.about.com/library/weekly/aa041301a.htm http://www.geocities.com/CapeCanaveral/Lab/7671/gregory.htm