  This paper describes the observations and the first data release (DR1) of the
ESO public spectroscopic survey "VANDELS, a deep VIMOS survey of the CANDELS
CDFS and UDS fields". VANDELS' main targets are star-forming galaxies at
2.4<z<5.5 and massive passive galaxies at 1<z<2.5. By adopting a strategy of
ultra-long exposure times, from 20 to 80 hours per source, VANDELS is designed
to be the deepest ever spectroscopic survey of the high-redshift Universe.
Exploiting the red sensitivity of the VIMOS spectrograph, the survey has
obtained ultra-deep spectra covering the wavelength 4800-10000 A with
sufficient signal-to-noise to investigate the astrophysics of high-redshift
galaxy evolution via detailed absorption line studies. The VANDELS-DR1 is the
release of all spectra obtained during the first season of observations and
includes data for galaxies for which the total (or half of the total) scheduled
integration time was completed. The release contains 879 individual objects
with a measured redshift and includes fully wavelength and flux-calibrated 1D
spectra, the associated error spectra, sky spectra and wavelength-calibrated 2D
spectra. We also provide a catalog with the essential galaxy parameters,
including spectroscopic redshifts and redshift quality flags. In this paper we
present the survey layout and observations, the data reduction and redshift
measurement procedure and the general properties of the VANDELS-DR1 sample. We
also discuss the spectroscopic redshift distribution, the accuracy of the
photometric redshifts and we provide some examples of data products. All
VANDELS-DR1 data are publicly available and can be retrieved from the ESO
archive. Two further data releases are foreseen in the next 2 years with a
final release scheduled for June 2020 which will include improved re-reduction
of the entire spectroscopic data set. (abridged)
