ולהיות מחמת חלישות הדור, אין כח בכל אחד ואחד להתענות כראוי לו

Moreover, since, due to the frailty of our times, not everyone is capable of fasting as he ought,

In Iggeret HaTeshuvah13 the Alter Rebbe cites the classical works of Mussar as to the number of fasts prescribed for each major sin, so that a penitent will be able to render himself as acceptable to G‑d after his repentance as he was before sinning.

לזאת עצה היעוצה, כמאמר רז״ל: כל השומר שבת כהלכתו, מוחלין לו על כל עוונותיו

the counsel offered follows the declaration of our Sages, of blessed memory,14 “Whoever observes Shabbat according to its law, is forgiven all his sins.”

כהלכתו דייקא

The term “according to its law” is used advisedly, for the Shabbat cannot be properly observed without a knowledge of its laws.

לכן מוטל על כל אחד ואחד, להיות בקי בהלכתא רבתי לשבתא

It is therefore incumbent upon every individual to master the “great law” of Shabbat.

The term “great law” echoes the expression in the Gemara15 regarding checking one’s clothes before sundown on Friday in order not to transgress a prohibition later. The laws of Shabbat thus not only inform us of what is prohibited, but also of how to avoid transgression.

וגם יזהר מאד שלא לשוח שום שיחה בטילה, חס ושלום

Also, be most careful [on Shabbat] not to indulge in idle chatter, G‑d forbid.

בהיות מודעת זאת ליודעי חכמה נסתרת, כי בכל המצות יש פנימיות וחיצוניות

For, as is known to the initiates in the mystical wisdom [of Kabbalah], all the mitzvot comprise an internal and an external aspect — the spirituality of the mitzvah, and the physical act which it requires.

וחיצוניות מהשבת היא שביתה מעשיה גשמיית, כמו ששבת ה׳ מעשות שמים וארץ גשמיים

The externality of the [mitzvah of] Shabbat is the cessation of physical activity, just as G‑d ceased making the physical heaven and earth.

ופנימיות השבת היא הכוונה בתפלת השבת ובתלמוד תורה, לדבקה בה׳ אחד

The internal dimension of Shabbat is one’s intention in the Shabbat prayers and during one’s Torah study, to cleave to the One G‑d,

כמו שכתוב: שבת לה׳ אלקיך

as it is written,16 “It is Shabbat to the L‑rd your G‑d.”

Underlying the cessation of labor on Shabbat is the concept of elevation. When a person rests from his labor at any time, the energy that had been vested in it rises and returns to its source within the soul. So, too, the cessation of labor and resting on Shabbat means that the soul, which during the week had been immersed in mundane activities, is uplifted “to the L‑rd your G‑d.”

וזו היא בחינת זכור

This [internal level of the mitzvah of Shabbat] is the element of “remembering”.

The Shabbat comprises two elements, “remembering” (zachor) and “observing” (shamor), reflecting the two commandments,17 “Remember the Shabbat day, to sanctify it,” and18 “Observe the Shabbat day, to sanctify it.” Elevating the soul on Shabbat through proper intent (kavanah) during prayer and Torah study, is an act of “remembering”.

ובחינת שמור בפנימיות, היא השביתה מדיבורים גשמיים, כמו ששבת ה׳ מיו״ד מאמרות שנבראו בהם שמים וארץ גשמיים

The inner dimension of the element of “observing” is refraining from speech about material affairs, just as G‑d ceased from the Ten Utterances through which the physical heaven and earth were created.

The external aspect of “observing” is refraining from active labor; the internal aspect of “observing” is refraining and resting from speech about material affairs.

כי זה לעומת זה כו׳

For19 “one opposite the other…” — speaking about material affairs on Shabbat is the inverse of the rest and elevation that a Jew secures on Shabbat, through prayer and Torah study.