John Gill Commentary Ezekiel 44:26

"And after he is cleansed, they shall reckon unto him seven days." — Ezekiel 44:26 (ASV)

And after he is cleansed From any sin or iniquity, failing and imperfection, that he has been guilty of at such seasons, in mourning for the dead, by a fresh application of the blood of Christ, which cleanses from all sin; typified by the water of separation, made of the ashes of the red heifer, by which those who were in this way ceremonially unclean were cleansed, (Hebrews 9:13Hebrews 9:14).

they shall reckon to him seven days ; that is, seven days shall be reckoned from the time of his cleansing, before he enters on public service again:

according to the old law, seven days were reckoned from the defilement to the purification; here seven more are numbered after the purification is made; and therefore, as Kimchi truly notes, this is a new law or rule, to be observed in after times.