Two collisions sit side by side here, and both run on ground the tradition itself honors. The fasting question (verses 18-22) comes from devout practice: voluntary fasts, typically Monday and Thursday, kept by the pious beyond the Torah's one commanded fast. Jesus answers from wedding law: the tradition exempts a bridegroom from reciting the Shema on his wedding night because joy overrides duty (Mishnah Berakhot 2:5). “Can the children of the bridechamber fast, while the bridegroom is with them?” claims that exemption for His disciples, and names Himself the Bridegroom, a role the Prophets give to God (Hosea 2, Isaiah 62:5).

The grainfield scene (verses 23-28) is unpacked in detail in this site's Matthew 12 note (the thirty-nine categories of Sabbath labor; plucking as reaping). What Mark alone preserves is the saying “The sabbath was made for man, and not man for the sabbath,” and it has a striking parallel in the tradition itself: “Shabbat is given into your hands, and you are not given to it” (Babylonian Talmud, Yoma 85b). Jesus argues on ground the sages would themselves articulate, then goes where no sage went: “therefore the Son of man is Lord also of the sabbath.”