From the verse “I shall fill the number of your days,” (“et mispar yamecha amalai”) the sages learned that God fills the years of pious individuals from day to day and from month to month, so that they are born and pass away on the same date. While there have been tzaddikim (pious individuals) in recent generations who were born and who passed away on the same date – for example, the Mittler Rebbe of Chabad, Rebbe Dov Ber, whose birthday and day of passing is on the 9th of Kislev – most of the tzaddikim were born and passed away on different dates. How then is this teaching from the sages relevant to all of us (since we know that the prophet says, “And Your nation are all tzaddikim”)?
Both a person’s birthday and day of passing have a special mazal. Mazal is the Divine source from which different powers flow to the soul. In Hebrew, the word mazal shares a root with the word, nozel, which means “to drip” or “flow.” On a person’s birthday, his “higher mazal” shines. This is the power and strength that the soul needs in order to perform its mission when it is sent from Heaven above to our world. As the sages say, before being born, the heavenly court address the soul and, “they make it take an oath: Be a tzaddik!”
On the day of a person’s passing, it is his or her “lower mazal” that shines. This is the day when all of a person’s Divine service ascends from below to above, with all the spiritual assets that he has acquired, clarified and rectified in this world. On that day, it becomes clear to the world what good fortune it had that this righteous person had descended to it and elevated it. The person who passed away can also wish himself “mazal tov” (good mazal) for all the spiritual assets that he has acquired.
Actually, a person’s birthday and day of passing express the “descent for the sake of a subsequent ascent” that defines the secret of the descent of the soul into this world. The birthday is the day of descent and the passing is the day of ascent. For a tzaddik who merits to fully realize the “descent for the sake of a subsequent ascent,” there is no difference between the two days. “Blessed are you when you come and blessed are you when you leave,” and, “May your exit from the world be free of sin as was your entry into the world.” Not only has the soul not been blemished, when a tzaddik leaves this world, he leaves with great spiritual acquisitions.
This is the meaning of the filling of the days of the tzaddik from day to day, when the higher mazal and the lower mazal—the mazal of the descent of the birthday and the mazal of the ascent of the day of passing—are perfectly unified. An allusion to this is in the word amalai, (אמלא) “I will fill,” from the verse cited above. The first and last letters of אמלא are alef, representing the birth and passing. Between them are the letters mem and lamed, which equal 70, “the days of our lives are 70 years.”
Chassidut explains that we do not have to wait to die to merit the ascent for which our soul descended to the world. The “descent for the sake of a subsequent ascent” is accomplished in and of itself, in the service of our souls in this world. Thus, the inner meaning of, “I will fill the number of your days” is daily: We must fill every day of our lives with illuminating, open and pleasurable service of God, by learning Torah, praying and performing acts of kindness. We are born anew every day. Our soul, which ascended to heaven while we slept, descends back into our body renewed and refreshed. When we go to sleep, it leaves our body, depositing our spirit once again in the hands of its Creator.
Chassidut explains that the soul-searching that we do before going to sleep has to remind us of the final account-taking that we will perform before we die (Hayom Yom for 6th of Kislev). The difference is that we have another day ahead of us tomorrow. A person who justifies (matzdik) his day by serving God throughout it, transforms it into a day worth living because of the “descent for a subsequent ascent” that has come to fruition through it. He is the tzaddik who is born and dies every day! And by doing so, he induces a fruitful unification of the two parts of his mazal, which evokes great abundance in all the worlds.
 Exodus 23:26
 Nidah 30b
 Psalms 90:10