A long time ago, the letters were used to designate about 2 years worth of futures contracts. 2 years, 12 months = 24 letters. In time, with rare exceptions a year was added and just 12 months of letter codes were used. ex: ESU1 =? E-Mini S&P for September 2021.
The months now used, in order are; F,G,H,J,K,M,N,Q,U,V,X and Z