Eventually Federal courts will decide whether the state has the right- and responsibility- to set rules for the primary, including a date. It seems to me that the Democratic National Committee is the one that is acting invalidly.

I have been secretary-tresurer or on the governing body of several societies, both in the United States and Canada. I don't know the rules for private societies in the UK, but the societies act set up by the state or provincial government imposes very strict rules on how a society is governed, including provision for election of officers at set times, rules on membership and set procedures for their implementation, annual audit by a certified accountant, etc., etc. Changes in procedures must be advertised in advance, and approved by majority vote. Expenditures must be approved by at least two signing officers. All of the requirements are in the Societies Acts, and thus are imposed on the 'private' societies by an arm of the government. A lawyer who is a member usually acts without charge to interpret the rules if the officers are unsure of some legal point.