Reb Tzadok Hakohen of Lublin elaborates on this principle, noting that, in Yechezkel, Shabbos is mentioned before Rosh Chodesh: "הַשַּׁבָּת יִפָּתֵחַ וּבְיוֹם הַחֹדֶשׁ יִפָּתֵחַ." Yet in Yeshayahu (סו כג) - "וְהָיָה מִדֵּי חֹדֶשׁ בְּחָדְשׁוֹ וּמִדֵּי שַׁבָּת בְּשַׁבַּתּוֹ" - Rosh Chodesh precedes Shabbos. And the same is true in Melachim (ב ד כג), where the Shunamite woman is asked "לֹא חֹדֶשׁ וְלֹא שַׁבָּת", and again Chodesh comes first.
The question is striking. Surely Shabbos possesses the greater inherent sanctity. Chazal describe it as "קביעא וקיימא" - its holiness was established by Hashem Himself from the moment of Creation. By contrast, the sanctity of Rosh Chodesh depends upon Klal Yisrael, for "יִשְׂרָאֵל דְּקַבְּעִי לְהוּ." Why, then, should Rosh Chodesh ever be accorded precedence over Shabbos?
The question is sharpened by the Gemara (זבחים צא ע"א), which states that when the Mussaf offerings of Shabbos and Rosh Chodesh coincide, the Mussaf of Rosh Chodesh is considered holier. Rashi explains that this is because Rosh Chodesh is called a "מוֹעֵד." At first glance, however, this seems only to deepen the mystery. Can the designation "מוֹעֵד" really elevate Rosh Chodesh above the sanctity of Shabbos, a day sanctified directly by the Ribbono Shel Olam?
Reb Tzadok answers with a profound insight. The differing orders reflect two distinct perspectives.
Yechezkel speaks from the perspective of Heaven. The Navi describes the gates that Hashem opens: "וּבְיוֹם הַשַּׁבָּת יִפָּתֵחַ." From the Divine perspective, Shabbos naturally comes first, for its holiness flows directly from Hashem.
Yeshayahu and Melachim, however, speak from the perspective of Klal Yisrael. From our vantage point, Rosh Chodesh precedes Shabbos. Why? Because "אָדָם רוֹצֶה בְּקַב שֶׁלּוֹ יוֹתֵר מִתִּשְׁעָה קַבִּין שֶׁל חֲבֵרוֹ." A person treasures most that which he himself has helped create. The sanctity of Rosh Chodesh is ours. Hashem entrusted the authority to sanctify time to Klal Yisrael, and the festivals themselves are fixed through our declaration. That partnership makes Rosh Chodesh uniquely precious from the human perspective.
This, Reb Tzadok explains, is also the meaning of the Gemara in Zevachim. The korbanos are brought by man. From the perspective of the avodah performed below, the Korban Mussaf of Rosh Chodesh takes precedence, for it celebrates a holiness that is realised through the participation of Klal Yisrael. Shabbos expresses the sanctity bestowed by Heaven; Rosh Chodesh expresses the astonishing dignity of Yisrael, whom Hashem empowered to sanctify time itself. (פרי צדיק, לחנוכה)
