The largest and most obvious ritual in this film is the ending scenes where the island people and Howie move toward the location for the sacrifice. They all dressed up in specific costumes. The regular townspeople all wore animal masks.
The more important figures like the Mayor wore costumes that symbolized figures in their ritual.
On the right we see Howie dressed as the Fool in the ritual. As the procession goes on they stop and have 6 men with swords interlock their swords together and make a sort of guillotine in the shape of a star.
This shape is likely symbolic of the Wicca star. The patrons of the ritual move their heads into the swords one by one almost like a trial with failure meaning decapitation.
Everyone moves through and eventually we move to another location. There we find Rowan tied to a pole. Officer Howie acts fast and free her. They both run off with Howie following Rowan. Rowan actually leads Howie into being cornered by the island people. The Mayor explains that this was all a trick to get Howie into coming there voluntarily for them to sacrifice him. From there they brake Howie’s knees and carry him to “The Wicker Man.” Here, the lock him inside and set it on fire while they sing.