Since the day is not the same when it turns on it will not be a bios problem.
IF by that you mean, that when the computer starts after having been deprived of AC power, then...
That would probably indicate that you have a bad CMOS battery.
Additionally, there is likely a setting in the BIOS that dictates the devices behavior after recovering from a power outage.
Usually this is set to "remain off" but it could be just as easily set to "power on".
IF the motherboard CMOS battery is bad, then when power is supplied, the BIOS is likely reverting to some default settings which likely includes date, time and startup behavior upon receiving adequate power.
You'll likely only see this when the computer is cut off from AC power by using the power supply switch, an in-line switch on a power strip or unplugging it from the wall.
Otherwise, the motherboard remains in a warm state when the computer is turned off normally and doesn't rely on the CMOS battery to maintain BIOS settings.