The nuclear envelope reforms during the phase of mitosis known as telophase.
During telophase, the separated sister chromatids reach the opposite poles of the cell, and the cell begins to prepare for division into two new cells. One of the key events in this phase is the reformation of the nuclear envelope around each set of chromosomes. This process helps to protect the newly separated genetic material and signals the end of mitosis. Additionally, the chromosomes begin to de-condense back into a less tightly packed form, making them less visible under a microscope, and the nucleolus reappears within the newly formed nuclei. Telophase is crucial for ensuring that each daughter cell receives a complete set of chromosomes.