officially proclaim a doctrine or law; make known by official publication
Today the government is expected to promulgate a new law restricting freedom of speech on the internet.
pro (forth)#vulgus (common people) (Latin)
Prom (promote)+ul+gate. So promoting something out of the gate, i.e. to make known by opening the gate (or opening declaration) to everyone out there. prom+ul+gate -> prom night will happen tomorrow was announced at school's gate