GG5: 〔元旦〕 New Year's Day; 〔新年〕 the New Year.
Kokugos have two senses.

Seems it's both.

@@ -15,0 +16,4 @@
+<sense>
+<pos>&n;</pos>
+<gloss>New Year's Day</gloss>
+</sense>

sankoku: 一月一日の朝。元旦。

元旦	797069	99.5%
歳旦	4269	0.5%

https://dictionary.goo.ne.jp/word/歳旦/
daijs:
《「旦」は朝の意》
    １ 1月1日の朝。元朝。元旦。《季 新年》「―や芭蕉たたへて山籠 (やまごも) り／蛇笏」
    ２ 「歳旦開き」の略。

daijs rather unusually points out specifically that 旦 = 朝.

sankoku and daijs both say morning, and both xref the far, far more common 元旦.

Every single news reference is to 歳旦祭, and a majority of twitter hits on "歳旦" are for 歳旦祭（さいたんさい）, which says to me that this is even more rare as a generic use of "new year's morning". Maybe not [rare], but comparatively [rare] next to 元旦.

歳旦祭 2514
歳旦   4269

@@ -12 +12,3 @@
-<gloss>New Year's Day</gloss>
+<xref type="see" seq="1260950">元旦・2</xref>
+<gloss>New Year's morning</gloss>
+<gloss>morning of New Year's Day</gloss>