As far as I understand, the junction object is hidden by Salesforce itself. I found an answer that advises to link objects by their names: BH : USA; Holiday : USA - New Year
But in this case we will select Holidays that have no connection with BusinessHours
BusinessHours.isWithin is not suitable, as it returns false if targetDate falls on a weekend day (Or any other non-business hour). I need to know exactly if the day is a holiday for a particular user associated with BusinessHours