  Observations of type II supernova early light, from breakout until
recombination, can be used to constrain the explosion energy and progenitor
properties. Currently available for this purpose are purely analytic models,
which are accurate only to within an order of magnitude, and detailed numerical
simulations, which are more accurate but are applied to any event separately.
In this paper we derive an analytic model that is calibrated by numerical
simulations. This model is much more accurate than previous analytic models,
yet it is as simple to use. To derive the model we analyze simulated light
curves from numerical explosion of $124$ red supergiant progenitors, calculated
using the stellar evolution code MESA. We find that although the structure of
the progenitors we consider varies, the resulting light curves can be described
rather well based only on the explosion energy, ejecta mass and progenitor
radius. Our calibrated analytic model, which is based on these three
parameters, reproduces the bolometric luminosity within $25\%-35\%$ accuracy
and the observed temperature within $15\%$ accuracy (compared to previous
analytic models which are indeed found to be accurate only to within an order
of magnitude). We also consider deviations of the early time spectrum from
blackbody, and find that the Rayleigh-Jeans regime is slightly shallower
(roughly $L_\nu \propto \nu^{1.4}$). This modified spectrum affects the
optical/near-UV light curve mostly during the first day when the typical
observed temperature is $\gg 10^4 ~^\circ$K. We use our results to study the
optical and near-UV early light curves from first light until recombination and
briefly discuss what can be learned from current and future observations. Light
curves generated using our calibrated model can be downloaded at
http://www.astro.tau.ac.il/~tomersh/.
