The first trading session to start is the Asian session. It is active from 11 pm to 8 am GMT. Participants tend to be most active in the morning. The Asian trading session closes on Friday and opens again on Monday.
The London session is the largest of all, accounting for over 30% of the world’s forex transactions. It runs from Monday through Friday from 7 am to 4 pm GMT, which is quite logically considered the official business hours in London.