The ordinal number of the months that have 31 days are one, three, five, seven, eight, 10, and 12. The months that have 31 days are January, March, May, July, August, October and December.
Ordinal numbers (or simply ordinals) are well-ordered sets. Ordinals are an extension of natural numbers and are different than integers and cardinal numbers. Ordinals may be added, multiplied and exponentiated like other types of numbers. Ordinals where first used by George Cantor in 1883 to help deal with infinite sequences and classifying sets during his work with trigonometric series. It is reported that he discovered ordinals by accident.