  We investigate the stellar populations for a sample of 161 massive, mainly
quiescent galaxies at $\langle z_{\rm obs} \rangle=0.8$ with deep Keck/DEIMOS
rest-frame optical spectroscopy (HALO7D survey). With the fully Bayesian
framework Prospector, we simultaneously fit the spectroscopic and photometric
data with an advanced physical model (including non-parametric star-formation
histories, emission lines, variable dust attenuation law, and dust and AGN
emission) together with an uncertainty and outlier model. We show that both
spectroscopy and photometry are needed to break the dust-age-metallicity
degeneracy. We find a large diversity of star-formation histories: although the
most massive ($M_{\star}>2\times10^{11}~M_{\odot}$) galaxies formed the
earliest (formation redshift of $z_{\rm f}\approx5-10$ with a short
star-formation timescale of $\tau_{\rm SF}\lesssim1~\mathrm{Gyr}$), lower-mass
galaxies have a wide range of formation redshifts, leading to only a weak trend
of $z_{\rm f}$ with $M_{\star}$. Interestingly, several low-mass galaxies with
have formation redshifts of $z_{\rm f}\approx5-8$. Star-forming galaxies evolve
about the star-forming main sequence, crossing the ridgeline several times in
their past. Quiescent galaxies show a wide range and continuous distribution of
quenching timescales ($\tau_{\rm quench}\approx0-5~\mathrm{Gyr}$) with a median
of $\langle\tau_{\rm quench}\rangle=1.0_{-0.9}^{+0.8}~\mathrm{Gyr}$ and of
quenching epochs of $z_{\rm quench}\approx0.8-5.0$ ($\langle z_{\rm
quench}\rangle=1.3_{-0.4}^{+0.7}$). This large diversity of quenching
timescales and epochs points toward a combination of internal and external
quenching mechanisms. In our sample, rejuvenation and "late bloomers" are
uncommon. In summary, our analysis supports the "grow & quench" framework and
is consistent with a wide and continuously-populated diversity of quenching
timescales.
