function Update-CalendarPermission {

	<#
	Update-CalendarPermission -Identify <mailbox> -User <xxx> -Permission <permission>
	#>

	param
	(
		[Parameter(Mandatory = $true,
			HelpMessage = "Enter a mailbox where you apply permission to")]
		[ValidateNotNullOrEmpty()]
		[string]$Identity,

		[Parameter(Mandatory = $true, HelpMessage = "Enter a user/group who will be granted the permission  syntax domain\xxx might be needed")]
		[ValidateNotNullOrEmpty()]
		[string]$User,

		[parameter(Mandatory = $true, HelpMessage = "Enter a valid permission set")]
		[ValidateSet("ReadItems", "CreateItems", "EditOwnedItems", "DeleteOwnedItems", "EditAllItems", "DeleteAllItems", "CreateSubfolders", "FolderOwner", "FolderContact", "FolderVisible", "None", "Owner", "PublishingEditor", "Editor", "PublishingAuthor", "Author", "NonEditingAuthor", "Reviewer", "Contributor", "AvailabilityOnly", "LimitedDetails", "Remove")]
		[string]$Permission
	)

	#Add Exchange 2010 Management Shell if needed
	if (! (Get-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010 -ErrorAction:SilentlyContinue)) {
		Add-PSSnapin Microsoft.Exchange.Management.PowerShell.E2010 -ErrorAction:Stop
	}

	$MBX = Get-Mailbox $identity

	$CalendarName = (Get-MailboxFolderStatistics -Identity $MBX.alias -FolderScope Calendar | Select-Object -First 1).Name
	$folderID = $MBX.alias + ':\' + $CalendarName

	if ($Permission -eq 'remove') {
		# special case, remove permission from user
		Remove-MailboxFolderPermission -Identity $folderID -User $User -Confirm:$False
	}
	else {
		$i = @(Get-MailboxFolderPermission -Identity $folderID -User $User -ErrorAction SilentlyContinue).count
		if ($i -eq 0) {
			# not in ACL, add permission
			Add-MailboxFolderPermission -Identity $folderID -User $User -AccessRights $Permission > $Null
		}
		else {
			# user is already in ACL, change permission
			Set-MailboxFolderPermission -Identity $folderID -User $User -AccessRights $Permission
		}

		# display new permission for $user
		Get-MailboxFolderPermission -Identity $folderID -User $User
	}
}