How can I change reservations to be allowed not earlier than 48 hours in before event takes place?
Now there is only an option to allow or not allow reservation current day or is there some options to set this up?
It is rellay important that I can set this up, I need it on all objects on my site.
If it's not possible can you customize it for me?

It can be very easy.
Look at the administrator/components/com_booking/helpers/booking.php.

There is row (651) looks like:

if (($config->bookCurrentDay ? ($timeDay > $service->fromUts) : ($time > $service->fromUts)) || $service->alreadyReserved>=$subject->total_capacity)

You can improve it e.g. to make time offset:

if (($config->bookCurrentDay ? ($timeDay > $service->fromUts) : (($time + DAY_LENGTH) > $service->fromUts)) || $service->alreadyReserved>=$subject->total_capacity)

I don't think I totally understand.
I have edited the file as you told me but nothing seems to affect the reservation time before you can book an object.

Should I edit line 651 like this?

if (($config->bookCurrentDay ? ($timeDay > $service->fromUts) : (($time + DAY_LENGTH) > $service->fromUts)) || $service->alreadyReserved>=$subject->total_capacity)

Or should I add something on the DAY_LENGTH parameter?

Hello, DAY_LENGTH is constant where is length of day in seconds.
This hack shifts current day for one day into future.
In property $time is output of PHP function time().

Yes it did effect as I wanted so now I can only book 48 hrs in advance, great!!!