Liturgically, feast days begin with First Vespers the evening before the calendar day.
For non-liturgical issues, the day follows the secular calendar.
The Day of the Lord (yes, we could call it the “Christian Sabbath” to distinguish it from the Jewish Sabbath), begins at First Vespers on Saturday evening. That’s how St John Paul II explained it in Dies Domini.
vatican.va/holy_father/john_paul_ii/apost_letters/documents/hf_jp-ii_apl_05071998_dies-domini_lt.htmlAd liturgicam consuetudinem enim dies festus incipit eiusmodi Sacris Vespertinis. Propterea Missae liturgia nonnumquam «praefestivae» appellatae, quae vero reapse pleno iure «festiva» est, dominici diei est, instante etiam celebrantis officio ut homiliam sacram habeat et cum fidelibus precationem universalem absolvat.The ICEL translation is more of an interpretation. He clearly says that feast days begin at Vespers, and that what happens in the evening is not “before the feast” but instead is the feast. He goes further to day that after Vespers, it (ie Saturday evening) is the “day of Sunday” or the “day of the Lord.”