  BL Lacertae had undergone a series of historical high flux activity over a
year, from August 2020 in the optical to VHE $\gamma$-rays. In this paper, we
report on optical flux and spectral variability of the first historical maxima
outburst event during October-November in g, r and i bands with the 1.26m
telescope at Xinglong observatory, China. We detected significant intranight
variations with amplitude rising up to $\sim 30$%, when the fastest variability
timescale is found to be a few tens of minutes, giving an emitting region size
of the order $10^{-3}$ pc, which corresponds to $\sim 100$ Schwarzschild radius
of the central black hole, likely coming from some jet mini-structures. Unlike
on intranight timescale, a clear frequency dependent pattern along with
symmetric timescales ($\sim$ 11d) of flux variation are detected on long
timescale. The spectral evolution was predominated by flattening of the spectra
with increasing brightness i.e., a bluer-when-brighter trend in 96% of the
cases. On the night before the outburst peak, the color indices clustered in
two distinct branches in color--magnitude diagram within a period of $\sim$ 6
hours that is connected to a hard-soft-hard spectral evolution trend extracted
from time-resolved spectra. Such trend has never seen in BL Lac or any other
blazars before to the best of our knowledge. The results obtained in this study
can be explained in the context of shock induced particle acceleration or
magnetic re-connection in the jet where turbulent processes most likely
resulted the asymmetric flux variation on nightly timescale.
