  The upcoming new generation of optical spectrographs on four-meter-class
telescopes will provide valuable opportunities for forthcoming galaxy surveys
through their huge multiplexing capabilities, excellent spectral resolution,
and unprecedented wavelength coverage. WEAVE is a new wide-field spectroscopic
facility mounted on the 4.2 m William Herschel Telescope in La Palma.
WEAVE-StePS is one of the five extragalactic surveys that will use WEAVE during
its first five years of operations. It will observe galaxies using WEAVE MOS
(~950 fibres across a field of view of ~3 deg2 on the sky) in low-resolution
mode (R~5000, spanning the wavelength range 3660-9590 AA). WEAVE-StePS will
obtain high-quality spectra (S/N ~ 10 per AA at R~5000) for a magnitude-limited
(I_AB = 20.5) sample of ~25,000 galaxies, the majority selected at z>=0.3. The
survey goal is to provide precise spectral measurements in the crucial interval
that bridges the gap between LEGA-C and SDSS data. The wide area coverage of
~25 deg2 will enable us to observe galaxies in a variety of environments. The
ancillary data available in each observed field (including X-ray coverage,
multi-narrow-band photometry and spectroscopic redshift information) will
provide an environmental characterisation for each observed galaxy. This paper
presents the science case of WEAVE-StePS, the fields to be observed, the parent
catalogues used to define the target sample, and the observing strategy chosen
after a forecast of the expected performance of the instrument for our typical
targets. WEAVE-StePS will go back further in cosmic time than SDSS, extending
its reach to encompass more than ~6 Gyr, nearly half of the age of the
Universe. The spectral and redshift range covered by WEAVE-StePS will open a
new observational window by continuously tracing the evolutionary path of
galaxies in the largely unexplored intermediate-redshift range.
