It is Sunday when I needn't go to school这是定语从句.
或
Every Sunday I needn't go to school.
或
It is on Sundays that I needn't go to school.强调句
It is Sunday when I needn't go to school这是定语从句.
或
Every Sunday I needn't go to school.
或
It is on Sundays that I needn't go to school.强调句