Direct answer
Objectives follow journal display order. Some depend on your choices; follow the active objective in the game.
Result
There are various ways to honor the gods: prayer, bloody offerings butchered on altars marked with pentagrams and donating to temples, to name a few. Skelligers, however, have their own, unique way of doing so. Since they value physical strength and warrior spirit above all else, they organize horse races in the goddess' name. Geralt decided to take part.