To Draw from the Kodesh:

The mitzvohs of the Torah are perfect and precise. It is impossible to altar them. When one adds on a section to his tefillin, he has not only failed to accrue a mitzvah, but has also committed a sin by adding something which he wasn’t commanded to do. All this is part of the rule “bal tosif u’ bal tigra” ― do not add to the mitzvohs or detract from them.

We learn this rule from Chava who told the serpent that Hashem had not only forbidden her to eat from the Tree of Knowledge, but also to touch it. Hashem had forbidden her only to eat from it, an she added on to the prohibition of touching it, bringing about her downfall. The serpent claimed that since nothing happened when she touched the Tree, nothing would happen if she ate from it. In the end, though, we know the consequences of her deed.

Tosefos Shabbos (Adding on to Shabbos) as opposed the Prohibition to Add to a Mitzvah.

On Shabbos and yomim tovim, it is a mitzvah to add on to them. On these days, we are commanded to add on to the kodesh from the chol (weekday) and to usher them in earlier than the time of their actual onset and to extend their departure times.

A few Achronim feel that one who adds onto Shabbos from the weekday, is also violating the rule of bal tosif [See Shu”t Besomim Rosh, in Kisei de’Harsana: 87, and Minchas Shlomo on Maseches Beitza 5a]. But the accepted rule is that one should add on to Shabbos and yomim tovim, while there is no such phenomenon with the other mitzvohs. Why is this so?

To Prevent Faltering

There are two explanations for this mitzvah. The first is cited by the Rishonim, who state that the exact time span of Shabbos is not clear, because a person cannot pin-point the precise time night begins and ends. Therefore it is a mitzvah to add on from chol to the kodesh, so that a person will not accidentally do melachos when Shabbos has already begun.

This obligates us to begin the preparations for Shabbos as early as possible and long before the shekia, so that at the last moment we won’t have things we must still do, and chalila end up doing melachos at an uncertain time.

Extending and Not Adding

Another reason for the mitzvah to add on to Shabbos, (unlike all other mitzvohs, stems from the ability of a Jew who is mekudash to extend the kedusha of Shabbos to the weekdays. This may be done even though it is actually chol, because the person who is saturated with kedusha may infuse it into a weekday. When a person adds from Shabbos to chol, he is not adding new time to the mitzvah, but is rather is extending the kedusha of Shabbos to the weekday. This, is an extension and not an addition. No other mitzvah has the capability of extending time, and every extension would be considered an addition. However the kedusha of Shabbos and the yomim tovim may be extended to another time. Thus the secret of Tosefos Shabbos is the extraction of the kedusha of the kodesh and its incorporation into chol.

Gedolei Yisroel ask: it is understandable that after Shabbos a person may extend the kedushas Shabbos which is already present in the world to the weekdays, but how, on erev Shabbos, when kedushas Shabbos isn’t present yet, can a person infuse kedushas Shabbos into a weekday?

Our Sages teach us that the essence of the being of every Jew is kedusha — holy. This is why he has the capability to connect to kedushas Shabbos and draw from it into the weekdays.

The value of one who adds on to Shabbos, even before Shabbos has actually begun is very special, in that it points out how a person can bring on himself an abundance of kedusha and light, which he has produced for himself. How can we pass up this amazing opportunity?