Hi, my name is Anderson Souza, I hope I can help you with your issue.
Can you confirm that you used the procedure below to show holidays in your Windows 10 calendar?
Has the problem started to occur recently? Do you know if it was after an update or installation of a new application?
https://www.groovypost.com/howto/add-national-h...
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.