In Judaism, and particularly on Pesach, the number fifteen is ubiquitous. The Mishnah (Middos 2:5) records that there were fifteen steps that led up from the Ezras Nashim to the Israelite Courtyard upon which the leviim would sing the fifteen chapters in Tehillim that begin with the words, "Shir Hamaalos" - "Song of Ascent". Rav Shimon Schwab suggests that since our daily prayers are patterned after the sacrifices in the Beis Hamikdash (Berachos 26b), it is appropriate to enter davening every day with fifteen Birchos Hashachar. Those blessings precede Pesukei Dezimrah which then concludes with the fifteen praises of Yishtabach. Yishtabach in turn introduces Birchos Keriyas Shema which culminates with the fifteen laudable attributes mentioned in Emes Veyatziv, setting the stage for the Amidah. In addition, the Pesach seder is held on the fifteenth day of Nissan during which we read the Haggadah, which has fifteen parts, punctuated by Dayenu, the popular song of gratitude with its fifteen stanzas.
What is the significance and message of all these fifteens?
The Gemara (Bava Metziah 85a) attests that "anyone who is a talmid chacham, and whose son is a talmid chacham, and whose grandson is a talmid chacham, the Torah will never again cease from his descendants ... from this point forward, after three generations, the Torah returns to its lodging." When three consecutive generations study Torah intensively and sincerely the Torah becomes indelibly entrenched in the identity of the family such that it is unlikely to ever be dislodged, as the pasuk states, "a three-stranded cord is not readily broken" (Koheles 4:12). However, Tosfos (Bava Basra 59a) restrict the Gemara's guarantee to the scenario where all three generations coincide and study Torah together. It is not the consistency of Torah study per say that ensures its continuity, but rather the intergenerational conversation and shared experience of a grandfather studying together with his son and grandson.
The Chida (Midbar Kedeimos) calculates that if Yitzchak was seventy-five years old when Avraham died, and sixty years old when Yaakov was born, it emerges that the intersection of all three Avos studying Torah together lasted for a period of fifteen years. Those fifteen formative years forged the bedrock of our mesorah which is passed from generation to generation on the fifteenth of Nissan through the fifteen stages of the Haggadah. This foundation also constitutes the essence of birchas kohanim, which consists of three pesukim with a total of fifteen words, corresponding to the three Avos and the fifteen years that they overlapped. Every Friday night, when this blessing is conferred upon children, it is with the intention that they develop into the next link in the chain of the mesorah that was established by virtue of the fifteen years that the three Avos interacted and learned Torah together.
Similarly, all three Avos are represented in the Mishkan. The acacia wood was taken from trees originally planted by Avraham in Beer Sheva (Breishis 21:33) and subsequently transported down to Mitzrayim by Yaakov (Breishis 46, 1 and Midrash Rabbah, Vayigash 94:4). The inauguration of the Mishkan was deliberately delayed until the first of Nissan, the birthday of Yitzchak (Midrash Rabbah, Pekudei 52:2). Indeed, according to the Ramban (Introduction to Sefer Shemos), the whole purpose of the Mishkan was to replicate and institutionalize the atmosphere of the ancestral home of the Avos. Therefore, fifteen different items were collected and used in the construction of the Mishkan (Shemos 25:3-7), to reflect that the transcendent and perpetual holiness created by the Avos was predicated upon the fifteen years during which they toiled in Torah together.
Just like the Mishkan was destined to become a dwelling place for Hashem in this world, as the pasuk states, "And they shall make Me a sanctuary and I will dwell in their midst" (Shemos 25:8), so too every Jewish home strives to be worthy of this distinction. To that end, the Gemara (Sotah 17a) teaches, "If a man and woman merit - the Divine Presence rests between them. But if they do not merit - fire consumes them." The words "man" - "ish" and "woman" - "isha" both contain the letters aleph and shin. The difference between them lies in the middle letter yud in "ish" and the final letter heh in "isha." These two letters can be joined to form the name of Hashem, yud-heh, but in their absence all that remains is the aleph and shin which spells "aish" - "fire." If the pair conducts their marriage properly their home can become the seat of the Shechinah, but if they fail, a destructive fire can consume them. It is instructive that the name of Hashem which the Jewish home aspires to embody is spelled yud-heh, which also has the numerical value of fifteen. In order to welcome the Shechinah into their home, the couple must conduct their relationship in a manner that is faithful to the legacy of the previous generations which was cemented by the Avos over a span of fifteen years.
At the time of Keriyas Yam Suf, all those who were present, even the maidservants, personally witnessed the Divine hand and proclaimed aloud, "This is my God and I will enshrine Him" (Shemos 15:2). However, only the Jewish people continued further and declared, "The God of my father and I will exalt Him." Rav Elya Meir Bloch (Peninei Daas) observes that for the maidservants the events of Keriyas Yam Suf were powerful but fleeting. They were maidservants before and they remained such afterwards. Only the Jewish people, who were the beneficiaries of a historical framework and tradition through which they could process and internalize the experience became permanently transformed. It was the recognition that "my God" is also "the God of my father" that perpetuated and preserved the euphoric jolt of emunah.
At the seder, as we attempt to reexperience the geulah ourselves and deepen our sense of emunah, we should simultaneously be cognizant that this is not only our story but we are connecting to "the God of our fathers." We do not approach Hashem as individuals but rather as the next link in a long and illustrious mesorah that has survived and endured for generations. Every day the kohen began the avodah in the Beis Hamikdash with the mitzvah of terumas hadeshen. He first removed the ash from the previous day's korbanos, which he then placed in a visible spot adjacent to the mizbeach, before commencing with the duties of the day. Rav Samson Raphael Hirsch explains that this is because every day presents an opportunity for a fresh start but we do so while being mindful of the sacrifices of the past. It is precisely because we invest and engage in the historical mission of the Avos that we will hopefully be equipped to write the next chapter in our collective story and address the challenges of tomorrow.