By Rabbi Berach Steinfeld
The Gemara in Yuma, daf lamed gimmel, amud bais discusses the topic of “Ein Mavirin al Hamitzvos.” Rashi explains that one who encounters a mitzvah should not leave it. We learn this out from “Ushmartem … hamatzos” and we read it mitzvos.
The Radvaz in Chelek bais, siman taf kuf chof tes says that the concept of not leaving a mitzvah is a DeRabbonon and the posuk mentioned above is an “asmachta” (a remez in the Torah.) The Levush and Taz and Primegadim in siman chof heh argue and hold that not leaving a mitzvah is a DeOraysa.
Rashi explains this by saying one should not leave a mitzvah because he may miss doing the mitzvah altogether. The Chasan Sofer, siman ches says that there are two reasons why one should not leave a mitzvahupon encountering it. One reason is that it is embarrassing to the mitzvah when you leave it, even if you do some other mitzvah. The second reason is that you end up delaying the mitzvah you encountered first.
In a case where one has two mitzvos to do and he encounters one of the mitzvos, he must do that one first. This is the case even if the other mitzvah comes more often as the rule of ein mavirin al hamitzvos pushes away the preference that the other mitzvah comes more often. The Chasan Sofer says that this would apply even if the mitzvah you encounter first is only a DeRabbonon; it must be done first.
The Magen Avraham in siman chof heh, seif koton daled says that a person encounters a mitzvah and could do it then but is not ready to do it at that time, he can skip the mitzvah and do the other mitzvah which is time sensitive to him. A practical application of this would be if Shimon picks up his weekday tallis on Shabbos by mistake. He can put it down and then take his Shabbos tallis instead.
What happens if Reuven is on his way to daven maariv on Chanukah and it is after the zman of lighting candles but he did not yet light his menorah? He should light the candles first as he would be passing the mitzvah of hadlaka. This is despite the fact that maariv and krias shema come around more often than candle lighting. We could say that his daas is not to light right now since he wants to daven first, so it would fall under the category of the Magen Avraham that he is not ready for the first mitzvah. We could argue and say that the Magen Avraham is talking about a case where he is not intent on doing the mitzvah that he encountered at all (for example, Shimon does not intend to put on his weekday tallis at all.) In the case of Reuven going to maariv, he wants to light the menorah, but not this second. Nevertheless, it would seem to be the halacha that he should light first and then daven maariv. A proof to this can be brought from a case where one picked out the tefillin shel Rosh before the shel yad. He must put on the shel rosh first because of ein mavirin despite the fact that he really wants to put on shel yad first.
Once Reuven starts lighting his menorah he should finish lighting the rest of the candles even if he is yotzei with the first candle and the rest are only a hiddur. When he is done he should then go out to daven maariv.