Sila, the first night on board is casual. Usually what you embarked wearing. Chances are, many passengers won't get their luggage in their rooms until later in the evening.
It seems that people traveling with young children or people that retire early enjoy the early seating. We've done both. I prefer 8:15 seating. Just our preference. I don't like having to rush back from the sun deck or excursions, quickly shower and get to dinner. It's more relaxing, for my family, with the later seating.
I believe formal night is the second night.
Do not miss the warm chocolate melting cake on the dessert menu.