To make a change to your Time Conditions for Bank / Public Holidays, navigate to your call route and select your "Out of Hours" Time Condition.
Add the Time which you'd like to apply the Out of Hours routing.
For example for 25th December.
Add a new condition and then select "Days of Month" 25, the select "Months" December on the left-hand fields, you only need to fill in the right hand fields if you are selecting multiple days of the month.
You can also add a new condition to apply separate call routing for Public Holidays to your normal Out of Hours routing.