As i understand it, you want to offer the Special Delivery Saturday service, which would cost an extra £2 ?
This would be difficult as you would have to turn it on every Thursday after 5 o'clock and then turn it off on Friday at 5 o'clock. I think maybe you should just add a £2 extra handling charge during these times and change the define text. Creating a new module entirely is a little trickier than copying the files over. The problem with turning on and off something is that say, they want to order on a Friday but have it delivered to a work address on Monday ? then you're a bit stuffed unless you have a unique Saturday delivery module to turn on and off.
To create a special Saturday delivery module, you would have to copy the files across to new names, rename the class inside the include file to the new names of the files, change all of the constants in the include file (which go into the database) and then add £2 somehow to the rates, change the text in the new define file and then either turn it on and off on a Thursday/ Friday or write a bit of php to do it for you.
It's not impossible, it's very easy for a normal php programmer, it's just a bit time consuming.
Philip (the bloke wot wrote the modules).