The first Plug In I recommend churches use for their WordPress Site is the All In One Calendar.  This is a great free plugin that you can use to publicize all your events.

Here are some of the best features that offers:

* Multiple View Formats: Display events in your choice of layout, including Month, Week, Day,Agenda, and Posterboard view. Lock to specific views or allow users to choose. I personally use it to create a monthly calendar on a calendar page and a list of upcoming events on the bottom of all my page.
* Recurring Events: The best thing about this calendar plugin is that you can create recurring events for worship services and other weekly events.
* Integration With Facebook: What makes this calendar widget different is that it integrates with Facebook. This means you don’t have to post events on your website and your Facebook page. It has a set by set tutorial on how to create a Facebook App, which allows you to automatically import your church’s Facebook page events into your website calendar. The one downside is that currently you can only import events from a Facebook page. Right now, you can export to a Facebook User. They hope to let you export to a Facebook page in the near future.
* Subscription: This calendar allows people to import / subscribe to your calendar to their personal one (Outlook, ical, Google Cal, Etc.)
* Location and maps: Add detailed location data, embed a Google Map directly into your event details, and even specify exact longitude and latitude co-ordinates.
* Customize Themes: Choose from a set of calendar themes or customize the colors and fonts to match the look of your website.

Best of all it is easy to learn how to use and has great support!