It starts at sunset. The reason for this is two-fold. First, the opening chapter of Genesis describes each day as “it was evening, it was morning, the first day” i.e. measuring time from the evening. Second, in the period before watches and clocks, the only way to mark time was by nature, and the setting of the sun was a universal sign that was recognisable to all. The Sabbath actually lasts for 25 hours – the reason being that extra time was artificially added by the rabbis at he beginning and the end, so that if one was in a rush or got confused and started or ended it a few minutes early, one would not be breaking the ‘real’ Sabbath, just the extra protective bit around it. It therefore does not go out at sunset on Saturday night, but slightly later, usually marked by the appearance of three stars.