Why not store each date in a HoHoA?
Example 
%dates= (
   2004 => {
      11 => [ 14, 15, 16 ],
      12 => [ 1, 2 ],
   },
   2005 => {
      1 => [ 4, 5, 6 ],
      2 => [ 18, 20 ],
   },
);

[download]
So simply pump your dates into this structur, write 3 loops to print that structure and you're done.
s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

In reply to Re: putting dates int year, month date by Skeeve
in thread putting dates int year, month date by Anonymous Monk

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