Day 24 endometrium demonstrates confluence of predecidua between multiple arterioles. Note that the thin layer of predecidua has not yet formed underneath the surface epithelium, which is a feature of day 25 endometrium.
In late secretory endometrium (day 23-27), the presence of predecidualization and its location is key to dating.
1 Mazur MT, Kurman RJ. Diagnosis of Endometrial Biopsies and Curettings. New York, NY: Springer; 2005: 12-19.
2 Nucci MR, Oliva Esther. Gynecologic Pathology: Foundations in Diagnostic Pathology. Philadelphia, PA: Elsevier: 2009: 202-4.