You'll need to begin promoting as soon as the details have been finalized. Once the time and place are known, write down all the details including: time, place, address, contact name and number, event details, costs if any. One very low cost way to advertise is to design a flyer printing piece and make copies on the church copier in black and white using brightly colored paper. If your budget allows, you can actually print large quantities at a lower cost than on your office printer when you use an online flyer printing company - your quantity will need to be more than 250 to save you money. These can be set out for church members to distribute. Make a point to hand them to any visitors who come prior to the event and invite them to attend.
Local newspapers are another great source and often have free forms of advertising for your event. You can ask to speak with the religion editor or a reporter and see if they would like to write a story about your event. Church events, especially fund-raisers benefiting a good cause, are often good human interest content for papers. Barring interest in a full-blown story, the paper will often have a community or church event calendar that they can list your event at no charge. Be sure to contact all the newspapers in your area as soon as your plans are finalized as space is limited.
Local radio stations also have community calendar announcements. Contact the local stations you would like to be advertised on and find out the requirements for submitting an announcement. You will likely need to submit your information in "spot" form. Write a brief paragraph of the event details based on the requirement standards you obtain for each station and be sure to submit it in time to air a week or two before your event.
Post your flyer printing on local information bulletin boards a week or two before the event at libraries, super markets, etc. You will probably need to repost your flyer printing several times, so check back at each location every couple days.
Other local churches are generally willing to put up your flyer printing on their bulletin board and also add your event to their announcements. Give other churches a call or email and let them know the details of your events. Ask if they have any upcoming events that you can share with your congregation as well.
Be sure to post the event information on your own church website. If you have an e-newsletter be sure you add the event to that during the previous month and weeks leading up to the event.
Once these steps are complete, you're done with promoting your event. Now all you have to do is focus on the preparation details!