Status: This project is not actively developed, but the code is "out there" in the Subversion repo for people to use if they want.
SF.net project page
Just put the files in a php directory on the web server and then use the following basic instructions.
We were using Lotus Notes 6.5. Some calendars existed back on 5.x, so I'm pretty sure the converter handles those formats, too. If you look at the php code you'll see it's written to also handle some customized calendars we had in Lotus. But that won't affect it's functionality for the personal calendars.
The index.php file tells the user how the calendar events will be converted or not converted