Rollover

Rollover is the day that trading in one contract switches to the next contract. This is often referred to as the far contract becoming the near contract.

Many futures contracts trade on a three month cycle with four contracts in any one year. The contracts expire in March, June, September, and December. The contract expiration date is usually on the third Friday of the expiration month. However, traders do not switch to the next month's contract on the expiration date, they switch on the rollover date which is eight days before the expiration date, a Thursday.

The letters that are used to represent the expiration dates 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

You can read more this here: Rollover Days