More than 2,000 oarsmen have spent months preparing for a centuries-old Thai royal tradition that will fill Bangkok’s Chao Phraya River with a fleet of intricately decorated barges on Friday.

The Royal Barge Procession is held in time for the annual Buddhist Kathin ceremony, which takes place after the end of Buddhist Lent. Buddhists offer kathin robes to monks who have remained in their monasteries throughout the three-month retreat in the rainy season.

The fleet, consisting of a total of 52 barges, will take King Maha Vajiralongkorn, Queen Suthida and members of the royal family to the Wat Arun Ratchawararam temple.

The 2,200 oarsmen from the Royal Thai Navy started their training in June, learning how to row the barges in unison at a navy dockyard before rehearsing in the river in late July.

The full dress rehearsals were performed twice in late October, drawing spectators along both sides of the river.

The rowers must master not only the precise movements required to maintain...