First Day Covers of the Regular Postage Issues of 1922-1935 by Jack V. Harvey (1991, Paperback)

I'm thinking of buying a first-day cover whose catalog price is 850 euro. A seller is prepared to sell it for 390 euros but his item bears the date of January 2nd, though the catalog gives Jan 1 as the date of issue. I asked the seller about the validity of his item and he explained that post offices were (and probably are) closed on Jan 1 and that consequently one must accept his "second date" as that of issue. But how come that catalogs give Jan 1 as the date of issue at all without providing any alternative?