  Nova Cas 2021 erupted on 18 March 2021, reaching naked-eye brightness when
passing through photometric maximum 53 days later. We describe our ~daily
monitoring of its evolution, covering the first 660 days since discovery. In
all we obtained 574 highly accurate photometric runs simultaneously in the BVRI
bands, and 110 Echelle high-resolution spectra.
  The multi-band photometric evolution of Nova Cas 2021 has been mapped in
detail, and the strict similarities to the proto-type very-slow novae HR Del
and V723 Cas are discussed. All three novae displayed multiple and
short-lasting maxima while for months lingering around a bright plateau,
leading eventually into the final decline and the nebular phase. The decline
for all the novae proceeded at the same pace Flux(V)=(t-to)**alpha with
alpha=-2.3
  Prior to the primary maximum, the emission lines of Nova Cas 2021 were
characterized by a slim Voigt profile, and after it all lines became much wider
and characterized by a broad central component superimposed on a ever broader
pedestal. This transition happened at the same time gamma-ray emission was
detected for a few days. Along the 7-month plateau, the line profile displayed
a reckless variability at all velocity scales in response to the ever changing
brightness of the nova during the secondary maxima. Upon leaving the plateau on
day +230, nebular lines appeared and the ionization degree quickly increased,
passing from FeII/Balmer/HeI through HeII/Bowen and then to
[CaV]/[NeV]/[FeVII]. After about day +550, the profiles stopped evolving,
freezing their aspect. At later epochs, the profiles of all emission lines
turned densely castellated, with all dents stable in radial velocity at 1 km/s
over the last four months.
