We report on a detailed analysis of the optical light curve (LC) of the symbiotic system BX Mon, the data of which were gathered from the literature. The LC covers the period from 1889 December to 2009 March, with a gap of no observations between 1940 March and 1972 February. The LC is characterized by strong oscillations of peak-to-peak amplitude from 2 to >3 mag. Before the gap, the fluctuations were modulated mainly by a period Pa= 1373 ± 4 d. After the gap, the dominant periodicity became Pb= 1256 ± 16. Higher harmonics as well as a few beats of the two major periodicities can also be identified in the LC. We identify one of the beat periods, Pr= 656 d, as the sidereal rotation period of the giant component of the system. The period switching that took place during the gap in the observations was possibly associated with a certain cataclysmic event, hints of which may be recognized in the LC in the first 11 yr after the gap.
We suggest that the origin of the major oscillations lies in periodic episodes of mass accretion from the M giant on to the hot component of the system. After the gap, they are correlated with the periastron passage of the system and therefore appear with the binary period. Before the gap, the oscillations appeared with the diurnal cycle of an observer on the surface of the rotating M giant, whose Sun is the hot component. The event of the period switching is possibly related to an intensive magnetic activity in the outer layers of the giant star.