Return to the Blue Lagoon

In 1897, Mrs. Sarah Hargrave, a widow, and two young children (one of whom is the son of the castaways from the original film) are cast off from the ship they are travelling on, because the ship's crew are infected with cholera. After days afloat, Kearney, a sailor who has been sent with them, tries to kill the boy because of his excessive crying. Sarah angrily beats Kearney to death with a harpoon and dumps his body overboard. The trio arrives at and is stranded on a beautiful tropical island in the South Pacific. Sarah tries to raise them to be civilized, but soon gives up, as the orphaned boy Richard was born and raised by young lovers on this same island, and he influences the widow's daughter Lilli. They grow up, and Sarah educates them from the Bible, as well as from her own knowledge, including the facts of life. She cautiously demands the children never to go to the forbidden side of the island.
When Richard and Lilli are about eight, Sarah dies from pneumonia, leaving them to fend for themselves. Sarah is buried on a scenic promontory overlooking the tidal reef area. Together, the children survive solely on their resourcefulness, and the bounty of their remote paradise. Years later, both Richard and Lilli grow into strong and beautiful teenagers. They live in a house on the beach and spend their days together fishing, swimming, and exploring the island. Both their bodies mature and develop, and they are physically attracted to each other. Richard lets Lilli win the child's game Easter egg hunt and dives to find Lilli an adult's pearl as her reward. His penchant for racing a lagoon shark sparks a domestic quarrel; Lilli thinks he is foolhardy, but the liveliness makes Richard feel virile.
Lilli awakens in the morning with her first menstrual period, just as Sarah described the threshold of womanhood. Richard awakens in the morning with an erection, and suffers a nasty mood swing, which he cannot explain. They then get into an argument regarding privacy and their late mother's rules. One night, Richard goes off to the forbidden side of the island, and discovers that a group of natives from another island use the shrine of an impressive, Kon-Tiki-like idol to sacrifice conquered enemies every full moon. Richard camouflages himself with mud and hides in the muck; meanwhile, Lilli worries about his disappearance. Richard escapes unscathed, though he is seen by a lone native. Ultimately, after making up for their fight, Richard and Lilli discover natural love and passion, which deepens their emotional bond. They fall in love, and exchange formal wedding vows and rings in the middle of the jungle. They consummate their new-found feelings for each other for the next several months.
Soon after, a ship arrives at the island, carrying unruly sailors, a proud captain, and his beautiful but spoiled daughter, Sylvia Hilliard. The party is welcomed by the young couple, and they ask to be taken back to civilization, after many years in isolation. Sylvia tries to steal Richard from Lilli and seduce him, but as tempted as he is by her strange ways, he realizes that Lilli is his heart and soul, upsetting Sylvia. Richard angrily leaves Sylvia behind in the middle of the fish pond, in plain view of the landing party. Meanwhile, a sailor ogles Lilli in her bath and drags her back to the house. He tries to rape her and steal her pearl, before Richard comes to her rescue. The sailor opens fire on Richard who flees. Richard lures the sailor to his death in the jaws of the shark in the tidal reef area. Upon returning, he apologizes to Lilli for hurting her and she reveals that she is pregnant. She tells him that if he wants to leave, then she won't stop him, but that she wants to raise their child away from civilization, and from guns. They decide to stay and raise their child on the island, as they feel their blissful life would not compare to civilization. The ship departs and the two young lovers stay on the island, and have their baby, a little girl.

Question: When do human sacrifices occur on the island?
Answer: every full moon