Hi!
Welcome to the Microsoft Q&A forum.
Easter might not be visible because the U.S. holidays schedule in Outlook does not always include all religious holidays. The list of holidays included in the Outlook program itself could differ based on the version and calendar in use.
The standard U.S. federal holidays you should expect to see includes the following:
New Year's Day — January 1
Martin Luther King Jr. Day — third Monday in January
Washington's Birthday (Presidents' Day) — third Monday in February
Memorial Day — last Monday in May
Juneteenth National Independence Day — June 19
Independence Day — July 4
Labor Day — first Monday in September
Columbus Day — second Monday in October
Veterans Day — November 11
Thanksgiving Day — fourth Thursday in November
Christmas Day — December 25
You may check this link on how to add Holidays in Outlook: https://support.microsoft.com/en-US/Outlook/calendar/add-holidays-to-your-calendar-in-outlook