Expert workers in finland...........

I hav been trying to collect info abt tax system in finland,but can't find enough.I tried to ask couple of my finnish friends abt the tax system,but even they said that they dont hav clear concept

My friend has a tax card estimating amount of 12000€ as her annual income.By the end of Nov,she already earned 11,900.Ofcourse,her next salary is going to b more than 100...........and she was asking if she has to make a new tax card n give it to her employer before she gets her next salary.
I couldnot help her with this ques.Is there any difference between making a new card before and after her next salary?What would u suggest?Plz....her next salary date is 23rd and she might hav to rush !
thank-u