Hello V and welcome to cruise-chat.
I must admit I'm confused too. But based on previous experience, the formal nights will be Sunday night, the first full sea day, and Thursday night, after you leave Cabo San Lucas.
There are several threads on Golden Princess in this forum but briefly, she is big, elegant, spotlessly clean, four great fresh water pool areas, large casino and spa and workout areas, lots of open deck space, wide variety of restaurants and entertainment venues, and a friendly and efficient staff.
It's a nice balance of sea and port days; and there is a large selection of shore excursions with something for every activity level. In my opinion, Puerto Vallarta is best for sightseeing; Mazatlan for shopping and dining, and Cabo for water sports.
One tip: best place for sailaway is the Terrace Pool, all the way aft. Get a snack at the Horizon Court buffet, grab a big fancy drink, and watch L.A. and Long Beach harbors fade into the distance.
Have a great cruise. By the way, my wife will be on the cruise right after yours.