The beginning of each new day was marked at sunset for thousands of years, so that was NOT a new thing. But to do 'labor' on a sabbath, or one of the feast high sabbaths would have been considered a sin. Christ had already had a run in with the religious community for doing what they considered 'labor' on the sabbath.
And the religious community did not consider Christ to be Emmanuel, or God with us, as that is what He was to be named. And since it was God that made the 'sabbath' which means 'rest', it was/is impossible for Christ to have sinned on the sabbath. And because at 'sunset' on Passover day began the first day of the days of unleavened bread it was a 'high' sabbath. Christ would have been placed into the tomb before the sun set so there would be no laboring on the 'high' sabbath. And it would have been from the moment He was placed into the tomb that would have started the 'three days and three nights' clock.
The end of 'three days and three nights' would have take place on the 'sabbath', not any part of the first day of the week. There was nobody there 'watching' on the sabbath, as we are told the women came before the dawn on the first day of the week and the tomb was empty.
Friday, Saturday, Sunday, Three days