Use PowerShell to create new ICS files

New-IcsEvent.ps1

function New-IcsEvent
{
    [OutputType([System.IO.FileInfo[]])]
    [CmdletBinding(SupportsShouldProcess, ConfirmImpact = 'Low')]
    param
    (
        [Parameter(Mandatory, ValueFromPipeline, ValueFromPipelineByPropertyName)]
        [datetime[]]
        $StartDate,

        [Parameter(Mandatory)]
        [string]
        $Path,

        [Parameter(Mandatory)]
        [string]
        $Subject,

        [Parameter()]
        [string]
        $Location,

        [Parameter()]
        [timespan]
        $Duration = '01:00:00',

        [Parameter()]
        [string]
        $EventDescription,

        [Parameter()]
        [switch]
        $PassThru,

        [ValidateSet('Free', 'Busy')]
        [string]
        $ShowAs = 'Busy',

        [ValidateSet('Private', 'Public', 'Confidential')]
        [string]
        $Visibility = 'Public',

        [string[]]
        $Category
    )

    begin
    {
        # Custom date formats that we want to use
        $icsDateFormat = "yyyyMMddTHHmmssZ"
    }

    process
    {
        # Checkout RFC 5545 for more options
        foreach ($Date in $StartDate)
        {
            $fileName = Join-Path -Path $Path -ChildPath "$($Date.ToString($icsDateFormat)).ics"
            $event = @"
BEGIN:VCALENDAR
VERSION:2.0
METHOD:PUBLISH
PRODID:-//JHP//We love PowerShell!//EN
BEGIN:VEVENT
UID: $([guid]::NewGuid())
CREATED: $((Get-Date).ToUniversalTime().ToString($icsDateFormat))
DTSTAMP: $((Get-Date).ToUniversalTime().ToString($icsDateFormat))
LAST-MODIFIED: $((Get-Date).ToUniversalTime().ToString($icsDateFormat))
CLASS:$Visibility
CATEGORIES:$($Category -join ',')
SEQUENCE:0
DTSTART: $($Date.ToUniversalTime().ToString($icsDateFormat))
DTEND: $($Date.Add($Duration).ToUniversalTime().ToString($icsDateFormat))
DESCRIPTION: $EventDescription
SUMMARY: $Subject
LOCATION: $Location
TRANSP:$(if($ShowAs -eq 'Free') {'TRANSPARENT'})
END:VEVENT
END:VCALENDAR
"@
            if ($PSCmdlet.ShouldProcess($fileName, 'Write ICS file'))
            {
                $event | Out-File -FilePath $fileName -Encoding utf8 -Force
                if ($PassThru) { Get-Item -Path $fileName }
            }
        }
    }
}

# Either create events one by one
New-IcsEvent -Path D:\temp -Subject 'Be hung over' -Location 'Bed' -StartDate '2018-01-01 09:00' -ShowAs Free

# Or use the pipeline for multiple dates
'08-15', '08-16' | New-IcsEvent -Path D:\temp -Subject 'Super important meeting' -Location 'Area 51' -Visibility Private -ShowAs Busy

# By using PassThru, you can attach those to emails as well
$attachments = ('08-15', '08-16' | New-IcsEvent -Path D:\temp -Subject 'Super important meeting' -Location 'Area 51' -PassThru).FullName

$Outlook = New-Object -ComObject Outlook.Application
$account = $Outlook.Session.Accounts | Where {$_.SmtpAddress -eq 'YOUR@MAIL.HERE' } # In case of multiple accounts
$Mail = $Outlook.CreateItem(0)
$null = $attachments.ForEach({$Mail.Attachments.Add($_, 1, 1, ([System.IO.Path]::GetFileNameWithoutExtension($_)))})
$Mail.SendUsingAccount = $account
$Mail.Subject = 'Have some meetings!'
$Mail.Body = @"
Hi there!

We agreed on the attached meetings.

Cheers!
"@
$mail.Display()

To have valid ics file there must be no space after the ':'