  The flat spectrum radio quasar 3C 279 was observed in an extremely high
activity state on June 16, 2015 (MJD 57189). In this paper, we investigate the
properties of this flaring episode in the high energy $\gamma$-ray and optical
bands using data from the \emph{Fermi}-LAT, SMARTS, and SPOL observations
during the period June 1-30, 2015 (MJD 57174-57203). The highest emission state
in the $\gamma$-ray band detected by the \emph{Fermi}-LAT exhibits a peak flux
of $\sim$ 2$\times$10$^{-7}$ erg~cm$^{-2}$~s$^{-1}$ which is more than 25 times
the flux level measured in the low activity state of the source. The temporal
analysis of the daily \emph{Fermi}-LAT light curve suggests that the giant
flaring episode has characteristic rise and decay times less than one day. The
optical daily light curves in B, V, R, and J bands also indicate the flaring
activity from 3C 279 with flux levels peaking for two days on June 16-17, 2015
(MJD 57189-57190). The discrete correlation function analysis indicates a time
lag of 1 day or longer between the $\gamma$-ray and optical peaks during the
flaring episode. The $\gamma$-ray emission is also observed to show a
harder-when-brighter behaviour whereas optical emission exhibits an opposite
behaviour. The $\gamma$-ray emission region during the flare is observed to be
very compact and is located close to the base of the jet. The degree of linear
polarization in the wavelength range 500-700 nm measured using SPOL during this
period is also highly variable with a peak of $\sim$ 30$\%$ one day after the
$\gamma$-ray flare. Near simultaneous $\gamma$-ray flux points show a linear
anti-correlation with the degree of polarization during the period of
$\gamma$-ray flare. The significant drop in the degree of linear polarization
suggests a sudden increase in the tangled magnetic field strength in the
emission region.
