I have a model class that is following
public bool Saturday{ get; set; }
public bool Sunday{ get; set; }
public string Holiday{ get; set; }
My Project has RequiredIf in it.
[Required]
public int SalesID { get; set; }
[RequiredIf("SalesID==1", ErrorMessage = "License is required.")]
public string License{ get; set; }
It shows error message 'License is required.' when License is left blank only if SalesID is 1. License cannot be blank if SalesID is 1.
For your code it should be something like
[RequiredIf("Sunday,Saturday",AllowEmptyStrings=false)]
public string Holiday{ get; set; }
It means if Sunday and Saturday are true you can allow Holiday property to be an Empty String.