This can depend on how you define singular and plural in this context.
There are many English people who say I'm going on me 'olidays, and it is common enough in my view to be recognised as idiomatic. It invariably refers to a person's annual custom of going away on holiday. At one time the only 'holiday' that ordinary people had were days such as Christmas, Good Friday, Easter Monday, Whit Monday etc. These were Holy Days - holidays.
When workers won the right to annual leave entitlement, they began talking about their 'holidays' since there was more than one day of holiday involved. So they began going 'on their holidays'. (I have not checked this with the OED but I feel sure this is the origin of this plural use.)
One senses that the plural form has been in decline for many years, and you were less likely, anyway, to hear this form among the well-spoken middle-classes, who pronounce 'oliday' with an H. They will say 'I'm going on holiday'.