What is Serial CBC? and what is the importance?

we have a patient whom diagnosis is unknown. the doctor requested for "Serial CBC". the patient has a high WBC count with neutrophilia and adequate platelet count in his first night of admission. the next morning, his CBC is all normal but at night, his WBC count became very low (beyond the normal value) with lymphocytosis and adequate platelet count. stab cells and some atypical lymphocytes were also reported. why is that and what are the possible complications?