Which one is correct?
Happy Teacher’s Day or Happy Teachers’ Day?
Go for “Teachers’ Day”, using the plural possessive teachers’.
where the plural possessive is used.
“Father’s Day” and “Mother’s Day”
are spelt with the singular possessive,
but this may be because we only have one mother and one father,
and these days are usually celebrated within families,
with either the father or mother being honoured.
But Teachers’ Day is a day honouring all teachers in a country collectively.