Because this year is a leap year, daylight savings time began a week and a half before Purim, bringing with it consequent issues regarding “early Shabbos” and the appropriate time for Maariv. The question of defining halachic day and night thus becomes very important.
My grandfather, Rabbi Chaim Zev Bomzer z”l, passed away three years ago right before Rosh Chodesh Adar. As a talmid in Yeshiva in the ‘50s and ‘60s, he learned under Rabbi Moshe Aharon Poleyeff z”l and was quite close to him. I found a discussion of this issue in my grandfather’s writings and the explanations and elucidations he himself heard from Rabbi Poleyeff. I would like to present them here, paraphrased by me for publication in this venue:
We find that there are several areas in Halacha that are contradictory when it comes to what is defined as day and what is defined as night. For example, there are opposing positions quoted by the Rema in Hilchos Niddah (Yoreh Deah 196:1). He writes that some say that once the community davens Maariv, even if this is before nightfall, a woman must wait to check for hefsek tahara until the next night, because now it’s already considered nighttime. But he says that others hold that she can continue to check until the actual night, even if the community started Shabbos earlier. The minhag, he says, is to be machmir l’chatchila like the first opinion.