The R.A.Q.  (Recently Asked Questions) 

Always: Thanks to Rav Shlomo Wahrman, Shlita.  (Please confirm all these answers with your local Posek!  These are just my own "quick" answers to students' classroom questions.)

Q1. Three Meals on Shabbat

Q2. Dimmer Switches on Yom Tov

Q3.  When to shake a Lulav

Q4.  How many times does one shake the Lulav in each direction?  3 (based on a Yerushalmi).

Q1.  Why do we have three meals on Shabbat? (Gavi W.)

The mitzvah to have three meals on Shabbat is based on a pasuk from the Mahn incident.  (It is important to note that Mahn incident is as much about Shabbat as it is about sustenance coming miraculously from God.)  In Shemot 16:25 the Torah says  

וַיֹּאמֶר מֹשֶׁה אִכְלֻהוּ הַיּוֹם, כִּי-שַׁבָּת הַיּוֹם לַיהוָה:  הַיּוֹם, לֹא תִמְצָאֻהוּ בַּשָּׂדֶה. 

"And Moshe said 'You (all) shall eat it today since today, Shabbat, belongs to God, today you shall not be found in the fields.

The triple use of the word "today" indicates three meals--celebrations--focusing on God's ability to provide food.  The third of these meals is referred to as Seudah Shelishit, or Shalis Sudis.  The Aruch Hashulchan says that this meal is either a Torah requirement or a Rabbinic law established by Moshe himself.    There is much Kabbalism surrounding this meal.  The most basic reason, I think, is the fact that Man's attempt to sanctify God (LiKadesh O'toh) ususally comes in threes (e.g. "Kadosh, Kadosh, Kadosh").  In fact the Rambam says that one should make Kiddush Darabanan at this meal just like we do at the second meal, although this is not the custom of our communities today.

A few laws concerning Seudah Shelishit:

Q2. May one use a dimmer switch to increase and decrease the brightness of an electric light on Yom Tov? (Josh P.)

No.  Now the details:

First, it's important to understand that the question has validity even though the change of electricity flow is not for the sake of cooking.  The need for light also falls under the rubric of "Tzorech Ochel Nefesh" and therefore is theoretically permissible on Yom Tov.

Second, I'm going to avoid the controversy of "Under which of the 39 prohibited creative works does  electricity fall?" and just assume the issue is lighting and extinguishing a flame.

Lowering a flame is prohibited on Yom Tov unless the food on which the flame rests will be ruined by the strength of the heat.  If there's no food on the fire, you cannot lower it, regardless of whether the stove/oven is electric or gas.  (So leave--or place--food on the flame to avoid the problem.  Yes, this leaves room for a little "sleight of hand," but I'm pretty sure it's acceptable.)  Therefore, lowering a light by dimmer is not permitted.  Perhaps there is room to allow this in cases were one's Yom Tov enjoyment would be destroyed by the bright light, like in a situation in which every light in every room in the house was left on full blast, and no one can sleep; however, I think that you should ask a Rabbi and describe the specific scenario when the situation arises.  (The Rabbi might invite you to sleep at his house for the rest of Yom Tov!)  This ruling is based on the Tosafot ("ViHamistapeik Mimenu") in Tractate Beitza 22a.

Increasing a flame: Rav Shlomo Wahrman tells me that Rav Ovadia Yosef prohibits increasing light with a dimmer, as well.  God willing, I will find the applicable responsa to discover his reasoning, but clearly there might be some more latitude for increasing the light with a dimmer on Yom Tov. Until then, Rav Wahrman's "No" will be my guide.

Three-way lights:  Ask an electrician how a three-way light works.  My guess is that when you change the setting, you are actually shutting down the current in one wire and turning it on in another.  This would be prohibited under all circumstances, on Yom Tov as well.

Q3. Why does Beit Shamai say to shake the lulav when we say "Anah Hashem Hoshiya Na" AND "Anah Hashem Hatzlicha Na" whereas Beit Hillel (and Rabbi Akiva) say to shake it only on "Anah Hashem Hoshiya Na"?  (See the Mishna and Gemara in Tractate Succot 37a - 38b.) (David K.)

Perhaps their argument is founded in the argument between Babylonian and Jerusalem Amora'im regarding the reason for shaking the lulav.  The Babylonian Tana'im seem to say it is an act of recognition and acceptance of God's control over the agricultural process, and Jerusalem Tana'im seem to say that we are empowered with the act of shaking which actually protects the agricultural process from physical and/or metaphysical forces that would damage it.  I believe "Hoshiya Na" (Beit Hillel) is a bow to the recognition of God's direct power (like the Babylonian Amora'im), whereas "Hatzlicha Na" (Beit Shammai) might indicate some successful measure that God allows us to take to enable us to control nature (the Jerusalem Amora'im).


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