Sundays (and solemnities) begin on the evening of Saturday (the previous day) with First Evening Prayer (Vespers). Therefore, this year the new liturgical year (2020-21)and Advent began on the evening of Saturday 28th December 2020.
Therefore, in places which have a Mass on Saturday evening lighting the first candle would be quite appropriate at that Mass.
I believe doing this is really more of a devotion rather than a mandatory liturgical rite. Therefore, I do not think we need to get overly concerned, within reason, about the right time.