Date that should be used for MEWA exception 85% CBA Testing

Section 3(40)(A) of ERISA provides that the term “multiple employer welfare arrangement’’ (MEWA) does not include an employee welfare benefit plan that is established or maintained under or pursuant to one or more agreements that the Secretary of Labor (the Secretary) finds to be collective bargaining agreements ("CBA").

For purposes of section 3(40) , an employee welfare benefit plan is “established or maintained under or pursuant to one or more agreements which the Secretary finds to be collective bargaining agreements” for any plan year in which the plan meets a certain requirements. One of the requirements is (generally speaking) that at least 85% of the participants in the plan are Individuals employed under one or more CBAs. 

My question - There is no explanation in the regulation or the preamble that states when during the year this 85% test should be conducted (i.e. first day, last day, etc.) . Does anyone have any guidance they can point me to or an opinion they don't mind sharing on this issue? 

Thank you.