Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a Sabbath day’s journey away. Acts 1:12 ESV
For the Jew, the Sabbath was entirely a day of rest when all work was forbidden. A Sabbath day’s journey is how far a person can walk on a Sabbath without violating the Law.
A journey was limited to 2,000 paces or cubits, or seven furlongs and a half. A cubit was eighteen inches; so a Sabbath day’s journey was rather more than half a mile.
The distance of a lawful journey on the Sabbath was not fixed by the laws of Moses, but the Jewish teachers had fixed it at 2,000 cubits. This measure was determined on because it was a tradition that in the camp of the Israelites, when coming from Egypt, no part of the camp was more than 2000 cubits from the tabernacle, and over this space, therefore, they were permitted to travel for worship. Perhaps, also, some countenance was given to this from the fact that this was the extent of the suburbs of the Levitical cities, Numbers 35:5.
Mount Olivet was only 5 furlongs from Jerusalem, and Bethany was 15 furlongs. But on the eastern declivity of the mountain the tract of country was called, for a considerable space, the region of Bethany; and it was from this place that the Lord Jesus ascended. (Albert Barnes’ Notes on the Whole Bible)