When 'out of season' refers to laws, it means an activity is temporarily banned, usually for conservation. Fishing for trout is out of season to protect their breeding period.
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Cultural Timing
Some 'out of season' practices are due to tradition, not law, like not wearing white after Labor Day in the USA. Wearing white is out of season once summer ends.
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Time-Specific Events
'Out of season' can also apply to events or festivals that happen only during certain times. Carnival is out of season in October.