Did the Rashbam Practice what he Preached Regarding Shabbat?

Question

Rashbam, on Bereishit (Genesis) 1:5, states:

ויהי ערב ויהי בקר - אין כתיב כאן ויהי לילה ויהי יום, אלא ויהי ערב, שהעריב יום ראשון ושיקע האור, ויהי בוקר, בוקרו של לילה, שעלה עמוד השחר, הרי הושלם יום א' מן השישה ימים שאמר הקב"ה בעשרת הדברות, ואח"כ התחיל יום שני, ויאמר אלהים יהי רקיע. ולא בא הכתוב לומר שהערב והבקר יום אחד הם, כי לא הוצרכנו לפרש אלא היאך היו ששה ימים, שהבקיר יום ונגמרה הלילה, הרי נגמר יום אחד והתחיל יום שני.

“And it was evening, and it was morning” – It is not written here, “And it was night, and it was day”, but rather, “and it was evening, and it was morning” – the first day became evening and the sun set, and it became morning, the morning following the night, for the dawn broke” – behold, one day was completed from the six that Hashem spoke of in the Ten Commandments, and afterwards began the second day: “G-d said, ‘let there be a firmament’”. The text does not come to state that the evening and the morning are part of a single day, for it only needs to explain how there were six days – that the morning broke and the night was finished: behold, one days was completed and another day began.

The Rashbam believed that the day begins in the morning, not the night (Understandably, this lead to controversy). He even states that this is the intention of the Aseret HaDibrot with regard to Shabbat-- that Shabbat should begin on Saturday morning.

Is there any documentation that the Rashbam actually kept Shabbat in this manner? Is there any documentation of the opposite?

And furthermore, would ruling in this manner constitute Zaken Mamrei (a rebellious elder)?

Answer 1

With all due respect, I think you've misunderstood Rashbam's commentary on this verse. He is not delineating when a day starts or ends, but indicating how six days passed. He maintains (1:4) that the first day of creation started in the morning; if so, how do we have six days of creation?^[1] He answers that even though we don't have six exact calendar days, we have six twenty-four-hour periods, from daybreak to daybreak.

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^{[1] This question only arises with the supposition that a calendar day begins at nightfall — or some time other than daybreak — so that's likely what Rashbam held.}