  We present a detailed spectral and timing analysis of Cygnus X-1 with
multi-epoch observations, during $2016$ to $2019$, by SXT and LAXPC on-board
AstroSat. We model the spectra in broad energy range of
$0.5\!-\!70.0\,\rm{keV}$ to study the evolution of spectral properties while
Cygnus X-1 transited from hard state to an extreme soft state via intermediate
states in 2017. Simultaneous timing features are also examined by modelling the
power density spectra in $3.0\!-\!50.0\,\rm{keV}$ . We find that during
high-soft state observations, made by AstroSat on Oct $24,\,2017$ (MJD
$58050$), the energy spectrum of the source exhibits an inner disk temperature
(kT$\rm_{in}$) of $0.46\!\pm\!0.01\,\rm{keV}$ , a very steep photon index
($\Gamma$) of $3.15\!\pm\!0.03$ along with a fractional disk flux contribution
of $\sim\!45\%$. The power density spectrum in the range of
$0.006\!-\!50.0\,\rm{Hz}$ is also very steep with a power-law index of
$1.12\!\pm\!0.04$ along with a high RMS value of $\sim\!25\%$. Comparing the
spectral softness of high-soft state with those of previously reported, we
confirm that {\it AstroSat} observed Cygnus X-1 in the `softest' state. The
lowest MAXI spectral hardness ratio of $\sim\!0.229$ corroborates the softest
nature of the source. Moreover, we estimate the spin of the black hole by
continuum-fitting method, which indicates that Cygnus X-1 is a maximally
rotating `hole'. Further, Monte Carlo (MC) simulations are performed to
estimate the uncertainty in spin parameter, which is constrained as
a$_{\ast}>0.9981$ with $3\sigma$ confidence interval. Finally, we discuss the
implications of our findings.
