רמב"ן — החדש הזה לכם ראש חדשים

Q — 'החדש הזה לכם ראש חדשים' — but Tishrei is the year's beginning ('בצאת השנה'). So how is Nisan first? And why does the Torah never use month names?

A — Ramban's deep chiddush: the Torah's original system was to count months by number from yetziat Mitzrayim — no names, just 'first month', 'second month', up to twelve — so that every utterance of a date is an act of remembrance. This parallels weekdays, which Jews count by Shabbat ('אחד בשבת, שני בשבת'), keeping mention of Shabbat constant. So 'ראשון' here means 'first to ge'ula', not 'first of the year'. The familiar Persian month-names (Nisan, Tishrei, Kislev, etc.) entered Hebrew only with the return from Bavel, fulfilling Yirmiyahu's prophecy ('לא יאמר עוד חי ה' אשר העלה את בני ישראל מארץ מצרים, כי אם חי ה' אשר העלה ואשר הביא מארץ צפון') — the second redemption now bears the memorial, so the names import a different exile, a different ge'ula. They appear in Tanach only in nevi'ei bavel and in Megillat Esther.