as salam alaikum
the sunset (Maghrib time) marks the beginning of the new twenty-four-hour period (Yawm) or "day". For example, at Maghrib of Thursday starts the night of Friday (Laylat al-Jumu'ah) and at Fajr time of the night of Friday starts the daytime of Friday (Nahar al-Jumu'ah) and it lasts until the next Maghrib. The same goes for all the others days.
With prayers for your success.