ki Sisa

The Posuk says that you should watch the Shabbos כי קדש היא לכם for it's holy to you.
Asks רבי, נחומק'ה מהורודנא that it seems the word לכם is extra. כי קדש היא because it's holy should be enough. Why do I need the word לכם?
The Posuk says, ושמרו בני ישראל את השבת לעשות את השבת - the Ohr Hachaim asks, what does it mean לעשות את השבת?
How can we make Shabbos? We don't make it, we keep it; 'ה made it.
He answers that when we add to Shabbos on friday, & keep it going longer, then we're adding some hours from friday & Sunday to Shabbos, & 'ה considers it Shabbos.
That's man made Shabbos.
The Medrash says that when the Posuk says את שבתתי תשמרו, it spells שבתתי without a Vav to teach us that whoever observes Shabbos to its fullest will have a Bracha that the 6 days of the week will be without toil.
The Chafetz Chaim brings down a Zohar that says a similar concept, that all six days of the week get their Bracha from Shabbos, so if we make the Shabbos longer, we are increasing our Bracha for the week.
So maybe that's why we need the לכם- to tell us that if we keep the Shabbos קדש, and as the Ohr Hachaim says, we wait all week for it to come, & as the Chafetz Chaim says, we get ready early & are ready when it comes, then it will be לכם, since the whole week will be toil free with increased Bracha for us because of our commitment to making the most out of Shabbos!