In the USA, we consider the beginning of the day to be at midnight. In Israel, is it only regarding religious things that the day is thought of as beginning at nightfall? In other words, would a nonreligious Jew think of the day as beginning at nightfall as well? and do the surrounding arab cultures also consider nightfall to be the beginning of the day?

Shalom, Thank you for your question. What you are asking relates to social norms, and use of language, (both important things themselves), as opposed to religious terminology. As you hint at, the “regular” use in Israel sees Monday night as the continuation and following of Monday day, and not as the period of nightfall preceding Monday morning (as opposed to the religious meaning). Even amongst religious Jews, in everyday speech, generally ‘Monday night’ refers to the night after Monday day (until midnight at least). At least when used in relation to things not so religious. To get around this confusion, in places when it isn’t so clear, such as on wedding invitations, the phrase, in Hebrew “Or L…” is used. This phrase (taken from the Talmud) indicates that we are talking about the religious usage, and referring to the beginning of the night preceding the following daylight (so Or L’Yom Sheini means ‘at the start of Monday night’ which is referring to what we call in English, [and in day to day Hebrew] as Sunday night). On the other hand, when we talk about “Shabbat evening” (Leil Shabbat in Hebrew) everyone, both religious and secular, understands that to be referring to “Friday night” – that is the beginning of the Shabbat which comes after sunset of Friday. Unfortunately, I’m not familiar enough with the surrounding Arab cultures to be able to help you out with that question. I hope this is of some help – Blessings.