Just think about different promotion techniques and platforms which you can do this. For instance: You can promote through word of mouth By displaying posters By posting about activities happening in the library online By asking neighboring businesses to display posters and leaflets for you etc.
Use Facebook as a base as customers are more likely to use this than Instagram or Snapchat. Instagram would be more appropriate for the gallery as it is a a more visual platform. List events on Facebook so people can search by date.