Hello bolaji96,
Ensuring the holidays are shown correctly in the Calendar app is important. If you haven't already, you'll want to update iOS to the latest version which is currently iOS 15.6. Updating iOS is a great step that can help with resolving this issue. You can find the steps for updating listed below, along with the following article.
- "Back up your device using iCloud or your computer.
- Plug your device into power and connect to the internet with Wi-Fi.
- Go to Settings > General, then tap Software Update.
- If you see two software update options available, choose the one that you want to install.
- Tap Install Now. If you see Download and Install instead, tap it to download the update, enter your passcode, then tap Install Now. If you don't know your passcode, learn what to do."
Update your iPhone, iPad, or iPod touch
Let us know if the holiday continues to appear incorrectly after updating!
Have a great day!