It may be three weeks earlier or later depending on weather, elevation, and latitude. See What's in Bloom and Predicting Wildflower Blooms for details. This event may occur between late February and mid April, usually in mid March. Annuals are highly dependent on rainfall. Massive and widespread displays occur only about once a decade, when the winter rainy season is both earlier and wetter than normal.
Good shows happen in localized areas every three or four years.
Seeing one requires being able to travel on short notice, and perhaps great distances. Death Valley may be spectacular in a year when Organpipe Cactus National Monument is poor. The high Mohave Desert may peak two or three weeks later than the lower elevation and more southerly Sonoran Desert. A good bloom may occur in a remote area and remain undiscovered. They are somewhat more dependable than the annuals, making a good show in about half of the years and peaking some time in March.