Based on the search results, there are several holidays mentioned for September 21, 2023. Here are the holidays mentioned in the search results:

• Columbus Day
• Indigenous Peoples Day
• National Kick Butt Day
• International Beer and Pizza Day
• Leif Erikson Day
• National Moldy Cheese Day
• World Post Day
• Curious Events Day
• Health and Sports Day (Japan)

Therefore, the holidays for September 21, 2023 are Columbus Day and Indigenous Peoples Day in the United States, and several other holidays mentioned in the search results.