Subnuclear vacuolation is a characteristic sign of

The most characteristic signs of secretory phase are found in the glands. Their epithelial cells develop spherical translucent areas between the nuclei and the basement membrane which contain the precursors of the glandular secretion and which persist until about the 21st day of the cycle. This characteristic appearance is called subnuclear vacuolation and is presumptive evidence of progesterone activity and therefore of ovulation.Reference: Shaw's Textbook of Gynecology; 16th edition; Chapter 2; Normal Histology