The “Golden Hour” is the period of time in which the plaster on which you’re painting your fresco is PERFECT. It is the moment when the plaster will take your paint and respond gratefully. It’s not saturated with water and it’s not too far along in the curing process (plaster doesn’t really “dry”). For anyone who doesn’t paint fresco I suppose this chemical and artistic fact could be a metaphor for “seize the day” or “strike when the iron is hot” or any other common saying we use to remind ourselves and others to take advantage of the moment.
I love the paintings of Piero della Francesca. Here is a website with a tour of his Legends of the True Cross in the chapel at Arezzo.