Hi Zum, I got one of these last week,played with it for hours before fitting it!
the rule of thumb is , press "set". and the days of the week, "m t w t f" flash,by pressing the up key "^" it will change to "sat sun",and by pressing select,you can go through the weekends programming.
It would seem that if its flashing you can either select, or "^" or "v" to change.
you can set the weekend the same as the week days.
As previuosly said by moving a jumper on he back you can change from 5/2 to 7 day working ,but you have to reset the thing, losing all your programming.
incidentally, I find that it actually reads lower than the real temperature and switches on or off when it is 0.5 degrees away from setting, ie if you set it to 20c, it will come on when below, and go off when it reaches 20.5c and then wont come on again until temperature drops to 19.5c.
hope this helps!