Our Gemara on Amud Aleph discusses a distinction between a Nazir who vowed his Nazirhood while he was Tameh Mes (ritually impure due to exposure to a corpse), versus someone who began his status of Nazir while pure, and then became impure via exposure to a corpse. Both must undergo the 7 days of purification with the ashes of the Red Heifer in order to start or restart counting the days of being a Nazir to fulfill their vow. However, in the case of the person whose Nazir status never began yet, as he made the vow while impure, the seventh day, when he completes his purification, can also count as his first day of being a Nazir. However, in the case of the person whose Nazir status was disrupted in the middle, he can only start counting on day eight after purification, The reason is that since he requires certain sacrifices to be brought on day eight, this holds back his resumption of Nazirhood until they are fulfilled.
Tosafos asks, but even the Nazir who was impure at inception still requires the setting of the Sun on day seven to become fully pure. Why then can he start counting on day seven? Tosafos answers that the impurity prior to the setting of the Sun is diluted and does not represent a barrier to beginning the count. Perhaps the reason why this impurity is seen as weaker is due to it not being dependent on human action, such as bringing a sacrifice. It is just a matter of time until the Sun sets.
Rav Tzaddok (Tzidkas HaTzaddik 12) understands this metaphorically. The first step of purification is human action, such as the Red Heifer or bringing sacrifices, but the final steps happens cosmically through God, symbolized in the setting of the Sun. The process of repentance first depends on resolve and behaviors. However, there also is a supernatural element where God renders assistance and completes the process begun in good faith.
As Chazal say (Shir Hashirim Rabbah 5:2), God says, “Make an opening like the point of a needle, and I will make an opening large enough for wagons to pass through.” By the way, because the English idiom is “eye of a needle”, I have heard rabbis mistakenly quote this Medrash as “eye of a needle.” However, notably, the Jewish idiom is “point of a needle” “חודו של מחט”, which is even smaller.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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