שב ואל תעשה

There is a concept in Halacha called שב ואל תעשה, which means that even though there is a positive commandment, an עשה, involved in the performance of a Mitzva, there are situations when you can sit back, שב, and not do the Mitzva. An example of this is how we do not perform the עשה of taking a לולב on Shabbat, because of the rabbinic concern that you might come to carry it in a public domain. One would think that the Torah commandment of taking the לולב would outweigh the rabbinic concern. Nevertheless, the law of לולב (as well as blowing שופר on Shabbat) is pushed aside because of this principle of שב ואל תעשה.