Davening Mincha after accepting Tosefes Shabbos:[1]

One may Daven Mincha even after accepting Tosefes Shabbos, and so is the widespread custom.[2] However, once one has accepted Itzumo Shel Yom [such as Davening Maariv] he may no longer Daven Mincha.[3]

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[1] Implication of Admur Seder Hachnasas Shabbos who allows Davening Mincha even after sunset; P”M 263 A”A 28 that if he did not say Barchu, he may still Daven Micnha; Mishmeres Shalom 26/23; Eretz Tzevi 1:60; Minchas Yitzchak 9:20; Piskeiy Teshuvos 263:41; See Admur 393:2 regarding the difference between accepting Tosefes Shabbos and Itzumo Shel Yom

Other opinions: Some write one is to be particular to Daven Mincha before accepting TYosefes Shabbos. [SSH”K 46:5]

[2] The reason: As accepting Tosefes Shabbos is only regarding the Mitzvah of Melacha and not regarding

[3] Admur 263:23; Michaber 263:15; Mordechai 296; Hagahos Maimanis 5 Reish