It can be started any time during the year. The best time to establish the plan is at your group health insurance plan renewal. Because most groups receive rate increases at renewal, this is the ideal time to allow employees to get enrolled prior to the new Plan Year.
It is also quite acceptable to start a new short Plan Year anytime. Say it's April, and you prefer a calendar Plan Year from January 1 to December 31. You can start a Short Plan Year, say on June1 and end the first Short Plan Year on December 31. After December 31 your Section 125 Plan will be on a calendar year from January through December.