Informal invite suitable for sending to your friends, family and colleagues:

Hi [name],
Have you heard about Software Freedom Day on Saturday 18 September? It's an event held all over the world to celebrate and raise awareness of free software and open source software. I'll be going to the event at the State Library of Victoria, so you should come along too!

There will be an range of speeches, workshops and activities from 11am to 4pm. one of the highlights will be a speech by [speaker name] on [speech topic]. The full list of activities is available from the website:

http://www.sfd.org.au/melbourne/

Look forward to seeing you there!

Cheers,
[your name]