left(a, n) := makelist(a[i], i, 1, n)$
right(a, n) := block([m: length(a)], makelist(a[i], i, m - n + 1, m))$

a: [ ]$
for year from 1900 thru 2100 do
   for month in [1, 3, 5, 7, 8, 10, 12] do
      if weekday(year, month, 1) = 'friday then
         a: endcons([year, month], a)$

length(a);
201

left(a, 5);
[[1901,3],[1902,8],[1903,5],[1904,1],[1904,7]]

right(a, 5);
[[2097,3],[2098,8],[2099,5],[2100,1],[2100,10]]