When was the Sanhedrin council convened?
The meeting times are not mentioned in the Bible, but I did find a reference in the International Bible Encyclopedia, which stated, "According to the Tosephta' Sanhedrin 7 1, the Sanhedrin held its sittings from the time of the offering of the daily morning sacrifice till that of the evening sacrifice. There were no sittings on Sabbaths or feast days."
As to when the council first started, it is believed to have its origins in the seventy elders selected by Moses (Numbers 11:16-24), but they are not mentioned as sitting in a council in the Old Testament.
The first mention of the council in the books of Maccabees as a ruling council of elders who had the high priest at their head. It was originally called the Gerousia, but later developed into the Sanhedrin. Josephus also mentions this council in his writings. Thus, the Sanhedrin, as known in the New Testament, arose from a ruling body of aristocratic elders during the Greek occupation of Palestine.