Index finger is extended outward and draws the letter "J" in the air.
First letter of the month.
Futures and options contracts are standardized for monthly expiration cycles and different hand signals evolved to signify each month. Like most signals, the monthly signals were developed based upon simplicity and sometimes based upon the letter of the alphabet which serves as an industry code for each month. For instance, the commodity codes are F - January, G - February, H - March, J - April, K - May, M - June, N - July, Q - August, U - September, V - October, X - November, Z - December.