Explanation:

- Advent typically signifies the arrival or coming of something significant. It often refers to the beginning of an event or the introduction of a notable occurrence.

- Option 1: Purpose refers to the reason for which something is done or created. It does not relate closely to the term advent.

- Option 2: Departure is the act of leaving, typically contrasting directly with arrival.

- Option 3: Arrival denotes the act of coming to a place or the start of something. This aligns closely with the meaning of advent.

- Option 4: Turn generally refers to a change in direction or a twist in a narrative. It doesn't directly relate to advent.

Option 3: Arrival is indeed the synonym of advent.