Tente isso,
<% date = Date.today %>
<% @employees = Employee.where("status = ? AND FORMAT(date_of_birth,'dd/MM','en-US') = ?", "Active" , date.strftime('%d/%m')) %>
Saiba mais sobre a função de formato
<% date = Date.today %>
<% @employees = Employee.where("status = ? AND FORMAT(date_of_birth,'dd/MM','en-US') = ?", "Active" , date.strftime('%d/%m')) %>