Temporal and energy coverage during the flaring activity from 2010 March 10 (MJD 55 265) to 2010 March 16 (MJD 55 271). Fermi-LAT data were accumulated during two-day time intervals to ensure significant detections of Mrk 421, and is depicted here with a blue band. For better visibility of the observations at UV, optical, and radio band, where the observation time is usually short and the covered frequency band is narrow, an additional 20 min in time and half a decade in frequency are included when displaying the results. The names of all the optical instruments are listed in Table 1.