As odd as it sounds, you’d create an exception to make the occassional Friday a working day. Assuming you have altered the calendar to make all Fridays non-working, open the calendar and select the Friday you’d like to work. Click the Exceptions tab in the Change Working Time dialog. Give the exception a name, click in the start or finish field to the right. Click the Details button. Click the Working times option and enter the working times for that day. Repeat as needed for other days.