Can one make an early Shabbat?
1. Yes, making Shabbat early is entirely acceptable.
2. Most people who do this, however, go about it incorrectly. Namely: one should daven Mincha before Plag haMincha, and then, after PHM, one can daven Arvit, i.e. the 7 brachot of the Amidah or "Shmone Esre". One can of course also say/sing Kabbalat Shabath, even though this is not a hova (requirement).
3. One most definitely may not say Kriyat Shema with the brachot before sunset, and preferably one should not do so before Tzeit haKochavim (the appearance of 3 stars). If one recites Shema before the appearance of 3 stars, one must repeat it later at night.
4. This means that if one wishes to make Shabbat early, one says Kriyat Shema later at night, and not before Arvit. (Being “Someh Geula leTefillah” is not a hova in this and similar cases – see Rav Hai Gaon quoted by Rosh, Brachot 1:1).
5. This is the almost unanimous opinion of all the Geonim and Rishonim. It is also logical: how can one say "HaMaariv Aravim" when the sun is shining in one's eyes?
10th of Tamuz, 5770 Tuesday, 22 June 2010