Why does the Pasuk say "b'Sish'ah la'Chodesh"?
Targum Yonasan, Rosh Hashanah 9a: To teach us that one must begin the fast before nightfall. 1
Berachos, 8b: It implies that whoever eats and drinks on the ninth of Tishri, it is considered as if he had fasted on the ninth and tenth.
Tosefes Yom ha'Kipurim.
What are the implications of "Tishb'su Shabbatchem"?
Targum Yonasan: It implies that a. One should begin Shabbas and every Yom-Tov before nightfall; 1 b.One should celebrate them with joy.
See Npeirush Yonasan and Nos'ei K'lei Yonasan.
In order to resolve "b'Sish'ah la'Chodesh", Rosh Hashanah 9a concludes that one must begin the fast before nightfall. Why is this unlike "on the 14th
Moshav Zekenim (27) #1: There, the Mitzvah to eat Matzah is written after "ba'Erev". Here, Iruy is written before "b'Sish'ah", which implies that it applies during the day.
Moshav Zekenim (27) #2: Here is different, for it says "[v'Inisem b'Sish'ah la'Chodesh] ba'Erev me'Erev Ad Erev [Tishbesu]" The verse implies that we fast on the ninth, and at night we cease Melachah.