Is religious activity responsive to the presence and activity of other religions? How do religious minorities persist and keep their children from converting? We investigate these questions by examining the extent to which the celebration of Hanukkah, a Jewish holiday that is celebrated in December, is driven by the presence of Christmas. Hanukkah celebration in the US is especially suited to address these questions; US Jews are a minority who account for less than 2% of the population, and conversion and intermarriage, which is estimated at over 40% (United Jewish Communities, 2000), are key concerns among American Jews. A key observation that motivated this work is that Hanukkah is a minor holiday in Judaism in general and in Israel in particular but it is one of the most celebrated Jewish holidays in the US. Hanukkah is often called the Jewish Christmas because American Jewish parents give their children gifts, like their Christian neighbours. Surveys we conducted in both Israel and the US confirm that Hanukkah is perceived to be much less important in Israel. This stark difference in the importance of Hanukkah in Israel (where Jews are a majority) and in the US (where Jews are a minority) suggests that the extent of Hanukkah celebration in the US may be driven by the presence of Christmas. With so many other differences between Israel and the US, however, one should be cautious drawing any interpretation from this anecdotal fact. Our strategy is therefore to look within the US, by comparing the behaviour of different American Jewish households. Our hypothesis is that Jews with children are more likely to be affected by the presence of Christmas, because Jewish parents might worry that their children would feel left out, intermarry, or convert. That is, Christmas, a fun holiday for children, induces Jewish parents to compete. Thus, if the presence of Christmas is important, we expect that Jewish parents will celebrate Hanukkah more intensively than Jews without children. To account for the alternative hypothesis that children induce more intensive celebration of all holidays regardless of Christmas, we use the intensity of Passover celebration as a control. To account for the alternative hypothesis that Hanukkah is simply a more fun holiday for children than Passover, we use a difference-in-differences * We thank Andrew Scott (the Editor), two anonymous referees, Manuel Amador, Nick Bloom, Doireann Fitzgerald, Avner Greif, Seema Jayachandran, Tim Guinnane and Izi Sin for many useful comments and suggestions.