Saint Serapion of Sidon, Bishop of Thmuis in Egypt.
St. Serapion was a friend of St. Anthony the Great and St. Athanasius, and became Bishop of Thmuis in the Nile delta in the fourth century. A learned man, he defended the Orthodox faith against the Arians and wrote a treatise on the divinity of the Holy Spirit; an ancient liturgical prayer book bears his name.
He suffered exile for the faith and reposed in peace. The Greek synaxarion keeps his memory on March 21.
Feast day
On the New Calendar, Serapion of Sidon is commemorated on March 21. Churches on the Old (Julian) Calendar keep the feast on April 3 of the civil calendar.
Also commemorated on March 21

Seraphim of Vyritsa
Venerable, Σεραφείμ της Βίριτσα
Venerable Seraphim of Vyritsa, elder and wonderworker of Russia.