Kovid is both correct and incorrect. You can use UTF-8 characters in the IDs including Kanji but the IDs may not start with a number. If you edit the IDs to add say, 'x' after the id=" and then regenerate the table of contents (both the ePub3 nav document and the ePub2 ncx document), your ePub is valid. The three lines below are the XHTML code, the reference from nav.xhtml and the reference from toc.ncx
.
Code:
<h2 class="section-title" id="x2月1日">x1月1日</h2>
Code:
<a href="chapter-3.xhtml#x2月1日">x2月1日</a>
Code:
<content src="Text/chapter-3.xhtml#x2月1日" />
On a different note, Adobe Digital Editions is not happy with that ToC and I am not enthused enough to find out why. Sigil, calibre viewer and Thorium on my computer and my Kobo ereaders are happy with it.
And here's another copy of your file where I changed the ID to use month/day which seems to make ADE 4.5 happy with using the ToC though the heading may end up part way down the page.