1. A method of creating a data structure that includes a plurality of relevancy scores, each indicating a level of relevancy of a keyword cluster related to an event category for a given time period relative to an event of the event category, comprising: identifying a calendar event associated with a device identifier; determining, by one or more processors, event parameters of the identified calendar event; determining, from the event parameters, a plurality of keyword clusters associated with the event parameters for serving content to a computing device associated with the device identifier; generating a relevancy score for each of the plurality of keyword clusters over a plurality of time periods relative to the calendar event, the relevancy score of a first subset of the plurality of keyword clusters being higher during a first time period than a second time period and the relevancy score of a second subset of the plurality of keyword clusters being higher during the second time period than the first time period; creating, for the event, a data structure including the plurality of keyword clusters and the generated relevancy scores for each of the plurality of keyword clusters over the plurality of time periods relative to the calendar event; storing, to a memory, the created data structure; receiving a request to serve content on the computing device associated with the device identifier; determining a time period between a time at which the request was received and a time at which the calendar event is scheduled; selecting, from the stored data structure, a keyword cluster from the plurality of keyword clusters based on the relevancy score of the keyword cluster for the determined time period and an event category to which the identified calendar event belongs; using, during the determined time period, the keyword cluster selected from the plurality of keyword clusters to select a content item in response to the request; and transmitting, to the computing device associated with the device identifier, in response to the request, the content item selected using the keyword cluster selected from the plurality of keyword clusters to cause the computing device to display the content item.