You know what the preparation day is, correct?
Food for Sabbath consumption is to be prepared in advance. The work of food gathering (like the manna see Exodus 16:23) was the most basic violation of Sabbath rest. The disciples were caught red-handed. And Jesus himself by implication. He cited "David and his companions". Quite an admission for a defense.
They were eating food
in the field which was lawful according to the scriptures. See the response of Jesus in....
Matthew 12:1-12
[1],
At that time Jesus went on the sabbath day through the corn; and his disciples were an hungered, and began to pluck the ears of corn and to eat.
[2], But when the Pharisees saw it, they said to him, Behold, your disciples do that which is not lawful to do on the sabbath day.
[3], But he said to them,
Have you not read what David did, when he was an hungered, and they that were with him;
[4],
How he entered into the house of God, and did eat the show bread, which was not lawful for him to eat, neither for them which were with him, but only for the priests?
[5],
Or have you not read in the law, how that on the sabbath days the priests in the temple profane the sabbath, and are blameless?
[6], But I say to you, That in this place is one greater than the temple.
[7], But if you had known what this means, I will have mercy, and not sacrifice, you would not have condemned the guiltless.
[8],
For the Son of man is Lord even of the sabbath day.
Note you may want to do a study on gleaning laws of the old testament (e.g. Leviticus 19:9-10; Leviticus 23-22; Leviticus 24-19-21 etc).
[9], And when he was departed there, he went into their synagogue:
[10], And, behold, there was a man which had his hand withered. And they asked him, saying, Is it lawful to heal on the sabbath days? that they might accuse him.
[11], And he said to them, What man shall there be among you, that shall have one sheep, and if it fall into a pit on the sabbath day, will he not lay hold on it, and lift it out?
[12], How much then is a man better than a sheep?
Why it is lawful to do well on the sabbath days.
Hope this is helpful
