Very early on the first day of the week, just after sunrise (Mark 16:2). All four gospels record the timing of the resurrection news. Just as the stone had symbolism so does the time. The resurrection is literally 'a new dawn'. As God began creation on the 1st day of the week, so He begins new creation. You see the resurrection was never going to happen on a Tuesday afternoon. Its very timing points us towards the start of a new era. An era in which men and women could become part of a new humanity initiated by a risen Saviour.
It is the reason that Easter Sunday is a great day on which to have baptisms - symbolising the beginning of new life - resurrection life in Christ. It is why it is a great day to preach the gospel and a great day to receive it.