The Krusty Krab Semi-Annual Charity Night was a made-up event set up by Mr. Krabs in the episode "Penny Foolish." It is semi-annual, meaning it is scheduled to take place again sometime in the future.
In the episode, Mr. Krabs thought that SpongeBob found a penny. Desperate for that penny, he made several costly and futile attempts to convince SpongeBob to give it to him. During the night of that day, he invited SpongeBob to a fancy movie and restaurant (only a fancy version of the Krusty Krab) for The Krusty-Krab Semi-Annual Charity Night. At the Charity Night, Krabs showed SpongeBob a film about being "penniless". SpongeBob stated that he would like to donate a penny if he had one. Krabs screamed in horror inside his shell.