SELECT Date FROM debate WHERE Num_of_Audience  >  150
List the dates of debates with number of audience bigger than 150
[Schema (values) (types)]: | debate |  people : people_id (text) , district (number) , name (text) , party (text) , age (text) | debate : debate_id (text) , date (number) , venue (text) , num_of_audience (text) | debate_people : debate_id (text) , affirmative (number) , negative (text) , if_affirmative_win (text);