The opening of a trading session (i.e. delivery of quotations in the terminal) is the night of Sunday to Monday, 00:00 terminal time (GMT +2).
Pay attention to the opening and closing time for specific groups of trading instruments. E.g., you can trade American stocks only from 16:30 to 22:59, i.e. during the American trading session.
See the specifications on our website (“Trading session” column) or in MetaTrader to learn more about trading hours for each instrument.