It is Happy Mother's Day / Happy Father's Day, because children have one mother, one father.
Although students may have several teachers, one for each subject, if they are conveying this individually to each of their teachers, use the singular form:
Happy Teacher's Day
If there is a banner put up by the students of the school, wishing all the teachers at the school a happy day, then use the plural form:
Happy Teachers' Day
Although students may have several teachers, one for each subject, if they are conveying this individually to each of their teachers, use the singular form:
Happy Teacher's Day
If there is a banner put up by the students of the school, wishing all the teachers at the school a happy day, then use the plural form:
Happy Teachers' Day
Comments
Students: We have free audio pronunciation exercises.
Happy Teacher's Day
- anonymous
Teachers'
- anonymous