This specification describes technologies relating to displaying online content. In general, one aspect of the subject matter described in this specification can be embodied in methods that include determining a timing penalty for a content item, the timing penalty based in part on weights of previous impressions the content item has been allocated during a plurality of intervals within a delivery period. The methods may further include determining a weight associated with a pairing of the content item and an impression, the weight based in part on characteristics of the impression. The methods may further include determining a score for the content item, based in part on the weight and the timing penalty. The methods may further include selecting one of a set of matching content items, based in part on the score for the content item, and allocating the selected content item in response to a request.