From GMO's original primer on schedules:
If there are multiple files with the same format in your schedules folder distinguished only by the comment at the end of the filename, the first in alphabetical order would be used at league creation then the game would move through the list as years progressed. If there were 3 such files named like below for a league created in 2006, the game would load them automatically for the league like this:
2006 - ILN_BGY_G162_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_A
2007 - ILN_BGY_G162_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_B
2008 - ILN_BGY_G162_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_C
2009 - ILN_BGY_G162_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_A
2010 - ILN_BGY_G162_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_B.. .
Now if I am reading this correctly and I were to have 5 different schedules that I wanted to rotate through and I want to start a league say in 1931 here is the order my schedules would be used
1931 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR1 OF5
1932 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR2 OF5
1933 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR3 OF5
1934 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR4 OF5
1935 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR5 OF5
1936 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR1 OF5
1937 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR2 OF5
1938 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR3 OF5
1939 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR4 OF5
1940 - ILN_BGY_G154_SL1_D1_T4_D2_T4_SL2_D1_T4_D2_T4_C_YR5 OF5
And so on.
Pretty easy actually
