Jesus certainly was not implying to his disciples that they would be breaking the Sabbath by fleeing Jerusalem if the siege came on that day. We know this from Mark 2:23-28. Jesus justified the disciples picking grain on the Sabbath as well as David eating the showbread when he was fleeing from Saul on the basis of the principle that the Sabbath was made for man, and not man for the Sabbath. The Sabbath was made to provide for man’s needs, not make a hardship on man. This is why Jesus could heal on the Sabbath and the man healed could carry his bed on the Sabbath (John 5:8). The reason Jesus told his disciples to pray that the siege did not come on the Sabbath day was because the city would be locked up on that day with guards hindering anyone from coming or going making it much more difficult to flee. Jesus was not concerned with the Sabbath command but with how the Jews would restrict the escape from the city because of the command.