Let us learn a posuk with Rashi

Shmot 23 11-12

The list of laws in Mishpatim includes a mention of the shmitta year.

"And in the seventh you shall leave it alone…." This is a reference to the working of the land. 'On the seventh year don't work the land".

The next verse is about the regular Shabbat: the seventh day of the week.

"For a six day period you shall do your work, and on the seventh day you shall desist…" 

Rashi explains (quoting the mechilta) as follows:
"Even on the seventh year, the weekly Shabbat shall not be uprooted from its place, you should not say: Since the entire year is called Shabbat, the weekly Shabbat should not be observed".

According to Rashi the verses contain a warning. One should not think that the weekly Shabbat is abrogated on the sabbatical year. If the verse has to clarify this point under the heading of shmitta information, it is probably accurate to say that the position rejected is a reasonable one from some point of view. What Rashi teaches in effect, is that the seventh day Shabbat is only one possibility for remembering the creation of the world and the fact that the entire seventh year is also called Shabbat indicates that the ordinary Shabbat (seventh day) is no longer necessary.

This notion has far reaching implications for our understanding of the notion of Shabbat in general. The fact that Hashem directed us to keep the Shabbat once each seven days does not mean that the world was created in "seven" daily days. It does not mean that we know the necessary meaning of the verses in Breshit that describe creation, and that we can accurately determine how much time the creation process actually took. Quite the contrary.

If Hashem tells us to keep the Shabbat once each seven days this means that that is the way that the Torah decided we could best remember the Shabbat of creation. However, if the Torah creates another pattern using the number seven; six years and then a seventh year, it is reasonable that this becomes the only way commemorate the Shabbat of creation.

There is certainly an important message. Shabbat can be expanded to include more and more of our real time. Though we are enjoined to work, there is no doubt that the free time we find ourselves with has to be used properly. The Torah tells us that we 
can turn more and more of our existing time and energy into Shabbat.

Perhaps Rashi is teaching that the Shabbat of the seventh year is really a much greater achievement than the seventh day Shabbat. However, since we have to return to the seven day Shabbat on the eight year it is not reasonable to forget the regular Shabbat even for the one year of the shmitta.