TRhe PBOK guide mentions :
1. Kickoff meeting is usually associated with end of planning ( Page 86, PMBOK Sixth edition)
2. Kickoff may occur at different times :
- For smaller projects - its at start of planning phase (since whole team performs planning and execution)
- For some larger projects - the team comes into picture once the whole planning is done. So here the team is usually available only in execution phase. So in this scenario - kick off will take place in execution phase
In my opinion : kick off can take place whenever the team involvement begins.
Its situational.
If nothing is mentioned : then we go with 'usual approach' at end of planning phase
(understanding from Page 86, PMBOK 6th edition)