Thea,
This is the link to the Post Office's mail dates. Military Mail is the second table down. I find this to be overly optimistic and suspect that it is the estimated time to a major base, not most end destinations. http://www.usps.com/holiday/shippingcalendar.htm?
I allow 4 weeks for Priority Mail and 6 to 8 weeks for Parcel Post. I try to have all of my Christmas packages in the mail the weekend before Thanksgiving. In my experience, some Priority packages will reach their destinations in a week and others won't get there until the New Year no matter what I do. I only ship special requests between Thanksgiving and New Year's.