FOSS4G NA is a great conference on open-source geospatial software. Folks from the Cesium team have been attending since before Cesium was released. This year, it is in May in Raleigh, North Carolina.
The conference is a great opportunity to show off the work you are doing with Cesium by giving a talk. It only takes a few minutes to submit a proposal. The early bird deadline for proposals is January 22, about two weeks away.