  The signal produced in neutrino observatories by the pair-annihilation
neutrinos emitted from a 20 $M_{\odot}$ pre-supernova star at the silicon
burning phase is estimated. The spectrum of the neutrinos with an average
energy $\sim$2 MeV is calculated with the use of the Monte Carlo method. A few
relevant reactions for neutrinos and anti-neutrinos in modern detectors are
considered. The most promising results are from $\bar{\nu}_e + p
\longrightarrow n + e^{+}$ reaction. During the Si-burning phase we expect 1.27
neutrons/day/kiloton of water to be produced by neutrinos from a star located
at a distance of 1 kpc. Small admixture of effective neutron-absorbers as e.g.
NaCl or GdCl$_{3}$ makes these neutrons easily visible because of Cherenkov
light produced by electrons which were hit by $\sim$8 MeV photon cascade
emitted by Cl or Gd nuclei. The estimated rate of neutron production for SNO
and Super-Kamiokande is, respectively, 2.2 and 41 events per day for a star at
1 kpc. For future detectors UNO and Hyper-Kamiokande we expect 5.6 and 6.9
events per day even for a star 10 kpc away. This would make it possible to
foresee a massive star death a few days before its core collapse. Importance of
such a detection for theoretical astrophysics is discussed.
