יכול מראש חודש
“תלמוד לומר “ביום ההוא
The Hagada tells us that
[we] could [have thought that something, presumably the Mitzvah of Hagada,] starts on Rosh Chodesh; if not for the fact that the Torah says
“[Relate to your son] on that day”.
Why on earth would I have thought that?
I think that the possuk I cited on T minus 17 (Deut. 16:1) is the answer:
א שָׁמוֹר, אֶת-חֹדֶשׁ הָאָבִיב, וְעָשִׂיתָ פֶּסַח, לַיהוָה אֱלֹהֶיךָ: כִּי בְּחֹדֶשׁ הָאָבִיב, הוֹצִיאֲךָ יְהוָה אֱלֹהֶיךָ מִמִּצְרַיִם–לָיְלָה.
1 Observe the month of Aviv, and keep the Passover unto the LORD thy God; for in the month of Aviv the LORD thy God brought thee forth out of Egypt by night.
Why would I have thought that the Mitzvah starts on Rosh Chodesh?
Because the Possuk says that Pesach is to be observed for the [entire] month of Aviv, which of course begins on Rosh Chodesh! The modifier in Parshas Bo serves to limit the Mitzvah of Maggid to that specific night after the 14th. But the original premise is true: The broader observance of Pesach, which includes the planning and preparation on all levels, does begin on Rosh Chodesh. This is the point of the special Chodesh reading: God Himself began telling Moshe to prepare for Pesach on Rosh Chodesh!
Now for many families, this is a no-brainer. Many homes began working towards Pesach two weeks ago. But many haven’t. Perhaps they don’t have to. Maybe there are no kids at home, or you are going elsewhere for YomTov. Even for those people, Pesach observance needs to begin this Shabbos.
As a PS, I will add that this facet of a month-long respect for a holiday that falls within that month exists elsewhere as well.
I am thinking of Adar. The Megilla refers to the salvation of the Jews have relevant to the month of Adar itself, not only to the 13th, 14th and 15th.
From the Book of Esther, Chapter 9:
כ וַיִּכְתֹּב מָרְדֳּכַי, אֶת-הַדְּבָרִים הָאֵלֶּה; וַיִּשְׁלַח סְפָרִים אֶל-כָּל-הַיְּהוּדִים, אֲשֶׁר בְּכָל-מְדִינוֹת הַמֶּלֶךְ אֲחַשְׁוֵרוֹשׁ–הַקְּרוֹבִים, וְהָרְחוֹקִים. כא לְקַיֵּם, עֲלֵיהֶם–לִהְיוֹת עֹשִׂים אֵת יוֹם אַרְבָּעָה עָשָׂר לְחֹדֶשׁ אֲדָר, וְאֵת יוֹם-חֲמִשָּׁה עָשָׂר בּוֹ: בְּכָל-שָׁנָה, וְשָׁנָה. כב כַּיָּמִים, אֲשֶׁר-נָחוּ בָהֶם הַיְּהוּדִים מֵאֹיְבֵיהֶם, וְהַחֹדֶשׁ אֲשֶׁר נֶהְפַּךְ לָהֶם מִיָּגוֹן לְשִׂמְחָה, וּמֵאֵבֶל לְיוֹם טוֹב; לַעֲשׂוֹת אוֹתָם, יְמֵי מִשְׁתֶּה וְשִׂמְחָה, וּמִשְׁלֹחַ מָנוֹת אִישׁ לְרֵעֵהוּ, וּמַתָּנוֹת לָאֶבְיֹנִים.
20 And Mordecai wrote these things, and sent letters unto all the Jews that were in all the provinces of the king Ahasuerus, both nigh and far, to enjoin them that they should keep the fourteenth day of the month Adar, and the fifteenth day of the same, yearly, the days wherein the Jews had rest from their enemies, and the month which was turned unto them from sorrow to gladness, and from mourning into a Yom Tov; that they should make them days of feasting and gladness, and of sending portions one to another, and gifts to the poor.
Mordechai defined not only the days of Purim as special and joyous, but the entire month as well.
This is a perfect source for the Talmud’s instruction (Taanis 29a) that
משנכנס אדר מרבין בשמחה
When Adar comes in, we increase in joy.
According to many, this joy does not end with Adar, but runs through Nissan as well.
I believe that the universal custom to omit Tachanun the whole month of Nissan is built on that as well.