From ArianeSpace press release:

Arianespace VV02 – Proba-V, VNREDSat-1 and ESTCube-1: Launch postponed by 24 hours

Evry, April 25, 2013

To carry out additional checks on the mobile gantry system used on the Vega launch complex (SLV), the European Space Agency (ESA) and Arianespace have decided to postpone the Vega launch VV02 for 24 hours. It was originally scheduled for the night of May 2 to 3.

Liftoff is scheduled for the night of May 3 to 4, 2013, at exactly:

11:06:31 pm (local time in French Guiana, on May 3, 2013)