There are so many ways! You could create an event on Facebook or LinkedIn. You could also reach out to traditional media, if it's a big enough event, or a good charity event. Many times radio and TV stations, or local newspapers, are always looking to visit events so they can promote their sponsors. Always contact promotions people to see if they are interested. If you’re looking to promote online, another site you could use is eventsnearhere.com. You can post free ads there, and each ad gets a QR code that you could print out on flyers etc.