Is one supposed to accept Shabbos before sunset?

The answer is in the affirmative, and the source is an interesting one.

The gemora in Rosh Hashana 9a cites the possuk that commands us to fast on Yom Kippur (Vayikra 23:32). The referenced words of the possuk are 'b'tisha l'chodesh b'erev', where on the one hand it appears that the fast day takes place on the 9^th of Tishrei, but on the other hand the possuk says b'erev', which means that it only commences on the 10^th at night. The gemora learns from this that one must begin fasting a bit before the 10^th of Tishrei. This concept is known as 'mosifin m'chol al kodesh' - adding to the holy day by sanctifying part of the adjacent weekday and is called 'tosefes shabbos'.

The gemora continues to prove that this applies to Shabbos and Yom Tov as well, i.e. one must add to the Shabbos by sanctifying the Friday preceding Shabbos and one must add to the weekday preceding a Yom Tov. [1]

We should note that although the Rambam holds that 'mosifin m'chol al kodesh'  only applies to the actual fasting on Yom Kippur and not to Shabbos and Yom Tov, many Rishonim [2] hold, as stated, that it applies to Shabbos and Yom Tov as well.

How we do practically apply this halacha?

The application of this halacha requires that one must refrain from performing melacha, as if it is Shabbos, even though it is not yet Shabbos. Prior thereto, we must know that there are several opinions amongst the Rishonim as to when Shabbos begins. The Mishna Berura points out [3] that many Rishonim hold that Shabbos begins when the sun dips below the horizon and consequently from this point onwards, one may not violate the Shabbos in any form.

Accordingly, if we are to add to the Shabbos and refrain from melacha it must be done before sunset, as from sunset onwards we must refrain from performing melacha even if we did not add to the Shabbos from our own aspirations.

Therefore one must accept Shabbos before sunset. The Mishna Berura says [4] that a few seconds are not enough for úåñôú ùáú and it appears that one should add a few minutes for úåñôú ùáú (this is done verbally or mentally, as explained later) and refrain from melacha from that point onwards.

[Even without the mitzvah of úåñôú ùáú there is a problem with sunset in various locations. This is because of difficulties in calculating the exact time of sunset when, as in some cases, mountains obscure the sun and other halachic considerations. Accordingly one should refrain from melacha prior to the last minute. Similarly, daytime halachos should be performed a few minutes before sunset in order to avoid this problem].

How does one “accept” Shabbos early?

One should accept the Shabbos verbally and say “I hereby accept upon myself the holy Shabbos or "   hareini mekabel alai shabbos kodesh", or one should at least say this mentally. It is also known that if one says bo'i challa in the lecha dodi, or if one says '   mizmor shir l'yom ha'shabbos'  one accepts the Shabbos, because one mentions the Shabbos in these verses. [5]

The accepted custom is that the lady of the house accepts Shabbos with her candle lighting even though she lights some time before sunset.

What am I to do if mincha is davened in my shul close to Shabbos?

Since one should accept the Shabbos some time before sunset, one must make sure to daven mincha early enough to conclude the sh’moneh esreh before sunset and subsequently accept the Shabbos. It may happen the chazzan is still in the middle of the repetition, but each individual should accept the Shabbos immediately after the private sh’moneh esreh if it is close to sunset. One cannot accept the Shabbos before mincha, because many opinions hold that subsequent to accepting the Shabbos one may not daven the weekday prayer. [6]

My shul davens mincha five minutes before sunset and as a result I conclude my sh’moneh esreh after sunset. This obviously prevents me from accepting Shabbos, so what am I to do?

It is advisable to daven mincha early and conclude mincha before sunset and accept the Shabbos. Some poskim [7] allow davening mincha on one’s own in order to accept Shabbos on time. However this ruling has not been accepted in many communities. One should nevertheless try at least to accept Shabbos mentally before sunset and by thinking “from now I accept the holy Shabbos”.