Mark 2:23

‘And it happened that he was going on the Sabbath day through the cornfields, and his disciples began, as they went, to pluck the ears of corn.'

What the disciples were doing in plucking the corn would have been seen as within their rights on any other day of the week, as long as they did not use a sickle (Deuteronomy 23:25), and it is not for that that they would be criticised. The problem lay in the fact that they did it on the Sabbath day and that what they were doing was seen as reaping and threshing corn, both forbidden on the Sabbath (Exodus 34:21). The Rabbis had at various times laid down a considerable number of regulations about the Sabbath in order to prevent it being violated and this was included among them. And it was not just a matter of being awkward. They genuinely believed that such activity could have awful consequences.