PowerShell function to configure the out of office reply message automatically for all the holidays in your calendar, provided set up the calendar by following this blog: https://www.msoutlook.info/question/show-holidays-as-busy-or-out-of-office

send-out-of-office-message-outlook-powershell.ps1

Function Configure-OutOfOffice-Message
{
    Add-Type -AssemblyName 'Microsoft.Office.Interop.Outlook'    
    $outlook = New-Object -ComObject outlook.application      
    $namespace = $outlook.GetNameSpace('MAPI')
    $Calendar = [Microsoft.Office.Interop.Outlook.OlDefaultFolders]::olFolderCalendar
    $folder = $namespace.getDefaultFolder($Calendar)    
    $Holidays = $folder.items |
      Select-Object -Property Start, Categories | Where-Object Start -ge (Get-Date).AddDays(-1) | Where-Object Categories -eq 'Holiday' 
    Foreach($holiday in $Holidays)
    {
        Set-MailboxAutoReplyConfiguration -identity alias -AutoReplyState Scheduled -StartTime $holiday.Start -EndTime $holiday.Start -InternalMessage "Thank you for your email. I am out of office today, I will get back you as soon as possible. Have a great day!" -ExternalMessage "Thank you for your email. I am out of office today, I will get back you as soon as possible. Have a great day!"
    }
}