So, i want to make a timer that will turn on/off a circuit. I want to turn on the connected device at 08:00 and stop it at 20:00, so the interval is 12 hours. Correct me if i'm wrong but i can't use IC 555(or another IC), i'll need some huge resistors/capacitors. Is there another way to build that other than using a microcontroller?
You could use a crystal clock and a bunch of clock dividers like the 4040. Should be accurate to a few seconds over a few days. You'll need some logic gates to give an output for 8 hours and then do a master reset after 24Hr. It's a bit cumbersome but if you don't want to use a micro then this might be worth considering
If you are not comfortable with, or for some reason constrained against, using a cheap, minimal microcontroller such as the ATTiny25, there are alternatives to the 555 that can give you the 12 hour timing you need.
Assuming familiarity with the 555 timer in astable multivibrator mode, with equal duration high and low defined by an R-C combination, consider the CSS555C Micropower Timer as a replacement.
The CSS555C contains an integrated capacitor, so RC time is set merely by a resistor. Also, the IC contains a 6 decade internal programmable counter. This means, it's time constant can be set to multiples of the equivalent time constant of the 555 timer.
Duration can thus extend from microseconds to several days if needed, without impracticably large resistance values.
You will however need a microcontroller to program the internal EEPROM for the counter - this can be done independently of the final timer circuit.