A social networking system maintains content items associated with various topics. Each topic may be associated with a start time and/or an end time defining a time range during which content items associated with the topic are most likely to be relevant to social networking system users. The start time and/or end time associated with a topic may be based on information received from a user of the social networking system or on the occurrence of an event. The social networking system computes a score indicating a predicted affinity of a viewing user of the social networking system for each content item eligible for presentation to the viewing user and adjusts the affinity score based on a start time and/or an end time of an associated topic. One or more content items are selected for presentation to the viewing user based on the affinity score associated with each content item.