The prophet Ezekiel begins his book of the same name "in the 30th year" (which likely referenced his age at the time - Ezekiel 1:1). It is at this time he receives his first recorded vision from God, known as the "wheel in the middle of a wheel" or "wheel within a wheel" vision.
Abraham was promised that if God found at least thirty righteous people in Sodom and Gomorrah he would not destroy the cities (Genesis 18:30).
Thirty and the Judges of Israel
Jair, one of the Judges of Israel delineated in the Bible, had thirty sons. He was wealthy enough not only to provide each of them with their own horse to ride, but also gave each of them a city for 30 total (Judges 10:4). Ibzan, another Judge of Israel, had thirty sons and thirty daughters (Judges 12:9). Samson, a Judge of Israel from 1085 - 1065 B.C., offered a prize of thirty sheets and thirty change of garments to thirty men if they answered a riddle within a week (Judges 14:11 - 14).
How is the number 30 related to mourning?
Both the death of Aaron and Moses was mourned by the children of Israel for 30 days (Numbers 20:29, Deuteronomy 34:8).
Additional information on the Biblical Meaning of 30
Joseph, a type of Jesus, was thirty years old when Egypt's Pharaoh placed him in charge over all that he ruled. King David also, when he began to reign over Israel, was thirty years old (2Samuel 5:4).
31 in Scripture
One of the Hebrew names of God is El, composed of the Hebrew letters Lamed (representing 30) and Aleph (representing 1), which totals to the number 31.
Josiah, considered the best king of Judah after Israel split into two nations, began his reign at the age of eight and ruled for 31 years (640 to 609 B.C.).
One of the most commonly occurring names in God's word is Moses, which appears in 31 books. One of most frequently mentioned women in Scripture is Rebekah (Isaac's wife), whose name appears 31 times. The book of Luke contains materials from 31 Old Testament books, the second highest in the New Testament.