What's past masters night and do only past Masters get to deliver ritual on a past masters night?

The supplied sources do not contain a definition or account of a "Past Master's night."

What the sources do address is the authority of a Past Master to take the chair and govern a lodge in the Master's absence. Under English regulations, if the Master is absent, the immediate or Senior Past Master present rules the lodge, though the Senior Warden congregates it 1. In American usage, following the tradition of William Preston and Thomas Smith Webb, authority in the Master's absence descends directly to the Senior Warden, who may then, as a matter of courtesy, offer the chair to a Past Master present after the lodge has been congregated 1. The degree of Past Master itself is described by Albert Mackey as an honorary or side degree attendant upon election to the office of Master of a symbolic lodge 1.

Because the library contains no passages describing the specific lodge tradition known as a Past Master's night, it does not record whether ritual work or the conferring of degrees on that occasion is restricted strictly to Past Masters.