SHANGHAI, Jan 4 (Reuters) - China’s onshore spot yuan finished its domestic trading session at 6.4628 per dollar on the first trading day of the year on Monday, its strongest such close since June 15, 2018. If the spot yuan retains all the gains at late-night close, it will have gained 1% against the dollar on the day, whoch would be its best day since Oct. 9, 2020. (Reporting by Winni Zhou and Andrew Galbraith; Editing by Kevin Liffey)