daknetworks.com

Single Export:

New-MailboxExportRequest –Mailbox company.holidays –FilePath \\SERVER01\PST\hol.pst -IncludeFolders “#Calendar#”

Single Import:

New-MailboxImportRequest –Mailbox foo.user –FilePath \\SERVER01\PST\hol.pst -IncludeFolders “#Calendar#”

Mass Import:

$users = Get-Mailbox -ResultSize Unlimited
ForEach ($u in $users)
{
     New-MailboxImportRequest –Mailbox $u –FilePath -IncludeFolders “#Calendar#”
}

NOTES:
https://4sysops.com/archives/how-to-add-holidays-to-the-exchange-calendar-with-powershell/