Whay are some holidays day specific, and some date specific. And who get to choose which is which?
You step into the stream, yet the water keeps moving…Now you are in the presence of abscence
IMO:
The fixed date holidays were anniversaries of special events: Fourth of July, Christmas, New Years Day, Vetrans’ Day (11/11 - Armistace (sp?) of WWI).
Presidents’ Day used to be Washington’s Birthday, but since he was born before England used the Gregorian Calendar but lived after they did, his actual birthday was in dispute. Since Lincoln’s Birthday was the same month, they were combined into a Monday holiday, which are quite popular with Americans. This was also done so that we wouldn’t just vote for a president whose birthday would make for a good 3-day weekend. 
Passover & Easter are determined by astrometric indicators, which is why they vary year to year as to which weeks they appear.
Memorial Day, Labor Day, Columbus Day, and Martin Luther King, Jr. Day fall under the popular Monday holiday rule.
Thanksgiving the odd holiday. I presume that they made it on a Thurday (in US) so that we could take the following day off and have a 4-day weekend.
Judges 14:9 - And [Samson] took thereof in his hands, and went on eating, and came to his father and mother, and he gave them, and they did eat: but he told not them that he had taken the honey out of the carcase of the lion.