Shortly after R’ Yaakov Kamenetsky’s second marriage, Shavuos fell on Friday. The rebbetzin prepared an elaborate dairy Kiddush, including an assortment of cheese dishes. R’ Yaakov had forgotten to tell her that it was his family’s minhag (custom) not to eat cheese on Fridays – and a minhag has the force of halachah.
As they gathered around the table that morning, the rebbetzin noticed that R’ Yaakov appeared preoccupied, but when she asked him about this he replied, “I just wasn’t prepared for such a beautiful Kiddush.” He did not want to tell her the truth, for this would have hurt her feelings. As the Torah relates, Hashem altered the truth somewhat so as not to cause any ill feelings between Avraham and Sarah (see Bereishis 18:13).
When the rebbetzin returned to the kitchen momentarily, R’ Yaakov turned to the three talmidim who were his guests and asked them to serve as a beis din to be matter neder (release him from his vow) so that he could eat the cheese dishes. He explained the halachah surely permitted this, for if he were to refrain from eating the foods, his new rebbetzin’s feelings might be terribly hurt – and he would never have accepted the custom had he known this in advance.
The rebbetzin returned to the room and R’ Yaakov recited Kiddush, partook of the cheese dishes, and praised his new wife’s culinary skills. She was very happy and had no idea of what had taken place during her absence. Some time later, R’ Yaakov informed her of his custom and returned to his practice of not eating cheese on Fridays. ~ 5 Great Lives