To submit an event so it shows up in the event calendar, follow the link below and enter the info you have.
This will put it in the site’s event calendar. However you want to make an announcement, you will also need to make a separate News post, and select the Announcement category. If you only submit the event to the calendar event submission form, it will only appear on the calendar but you will not get the plus of having people see an announcement on the front page that it is taking place, so you definitely want to do both.
As with the News, events must be approved after submitted, before they show up in the calendar. You must be a registered member of the site in order to post.