When one has free time; a spare or idle moment (used to ask for help or make plans).
暇なときに映画を見に行きませんか？
Would you like to go see a movie when you have free time?
暇なときは読書をして過ごすことが多い。
When I have free time, I often spend it reading.
暇なときには散歩に出かける。
I go for a walk when I have spare time.
暇なときがあれば、手伝ってくれると助かります。
If you have some free time, I'd appreciate your help.