ויקרא, פרק ט״ו, פסוק כ״ט

After a woman completes seven clean days following an irregular flow, a final step is required to achieve complete atonement and allow her to return to the holy areas of the Tabernacle [ביאור שטיינזלץ]. She must offer sacrifices to finalize her purification process.

These offerings are governed by specific rules. They must be presented exclusively during daylight hours and never at night. The sacrifices must also be specifically set aside for her current situation. If she had previously designated birds for a past state of impurity, they cannot be repurposed for this occasion. Furthermore, when selecting the two required birds, they must be of the exact same species. She cannot offer a mixture of one turtledove and one pigeon [אדרת אליהו].

An unusual grammatical feature in the command to bring the offerings points to a special leniency [אבן עזרא, מנחת שי, פירושי רד צ הופמן]. Usually, multiple sacrifices must be presented at the same time. However, in this situation, she is permitted to bring her offerings gradually. If she is unable to provide both birds at once, she may bring the sin offering first and complete the process by bringing the burnt offering at a later time [העמק דבר].

Finally, presenting these sacrifices at the entrance of the Tabernacle carries a strict prerequisite regarding her physical purification. In order to approach the holy space, she must have already immersed in a ritual bath, and the sun must have set following that immersion. This requirement teaches that her immersion must take place during the day, prior to the actual day she brings her offerings [חזקוני, אדרת אליהו].