Fahime,
In the circuit, C5 filters low frequencies and C4 filters high frequencies. The 555 makes a high frequency spike on the supply when it switches and the electrolytic capacitor C5 has too much inductance to be a good filter at high frequencies.
20 hours is too long a timing period for a 555. The timing capacitor must be huge and have extremely low leakage current. An electrolytic capacitor has way too much leakage current.
Use a CD4541 oscillator/timer instead because it has a digital divider chain for long timing periods. It is low power Cmos so it needs a transistor on its output to drive a relay.
Pin 2 of a 555 has a threshold voltage that is at 1/3rd of the power supply voltage. The timing begins when the voltage on pin 2 is reduced to a voltage slightly lower than 1/3rd of the supply voltage.
Pin 6 has a threshold voltage that is at 2/3rds of the power supply voltage. When the capacitor charges to a voltage slightly higher than 2/3rds of the supply voltage then the discharge begins.
C1 is the timing capacitor. The circuit does not need C2. C2 is supposed to be the START switch.