שמור וזכור בדבור אחד נאמרו (לכה דודי)
It is well known that in the first recitation of the 10 Commandments (Shmos) in regards to Shabbos, the word זכור is used and in the later recitation (Devarim) the word שמור is used. Therefore, according to the order in the Torah, the phrase should be זכור ושמור (in Lechah Dodi).
One might be tempted to explain that the poet (שלמה הלוי) placed the words in this order since needed the letter ש to be the first letter of the phrase in order to have his name spelled out (in the first letters of each stanza of the poem). However, this is difficult to accept that a Torah scholar would place what should come first into the later position for a personal benefit.
Since the Shelah HaKodesh says the author of the poem לכה דודי is a Tzadik, who wrote the poem with Ruach HaKodesh, Rabbi Epstein wishes to propose an understanding of why the word שמור was listed first. In addition, the poet could have listed the phase שבת זכור ושמור and then, the letter ש would also have appeared first in the stanza.
Further, an external reasoning would also indicate that שמור should be listed later. When one gives someone a gift, and then the gift is misplaced, at the time that the person would give a second present they would mention that an additional level of care should be provided to guard the gift against being misplaced. Shabbos was a gift given on Har Sinai from HaShem to the Jewish people (see Gemara Shabbos 10b) which is indicated by the word זכור which implies the giving of a gift. And, as known, subsequently the Gatherer of the Sticks violated the Shabbos in public (as explained in Parshas Shlach), equivalent to misplacing the gift (or lessening of its importance). And, then in the plains of Moav at the end of the forty years in the desert, when Shabbos is again mentioned, the word used is שמור to emphasize a greater respect and care should be taken with this gift.
According to the Medrash (as brought by Ramban in Parshas Yisro), Rabbi Nechunyah ben Hakanah (one of the great Tananim) stated that the word זכור implies daytime, and the word שמור implies nighttime.
Therefore, the author, a great Kabbalist, composed this poem for recitation at the inception of Shabbos at Friday night. And thus, the order within the poem lists שמור first, since the holiness of the night of Shabbos occurs prior to the holiness of the day of Shabbos.
