I’ve got the same problem. For me, this is fixed by;
“changing the calendar-theme to another and restore the original again”.
* * *
I wondered if this touches the essence, my situation was as follows:
My site is located in a rental-server for the release version (R) and also in a local MAMP environment for development (L).
Both contents (files, DB entries, etc.) are identical, I believe:), but the output HTMLs are not identical like;
(R) <tdclass="ai1ec-today">…</td> (NO blanks between ‘td’ and ‘class’!)
(L) <td class="ai1ec-today">…</td> (OK)
Therefore, in (R), the part seems to be treated as “an unknown element”. That is displayed at the wrong place (in my month-view mode, today’s date# displayed just above the calendar table, and the week in the calendar has only 6 days).
PHP/JS version differences or TWIG caching inconsistency may cause the issue.
I cannot fix the problem in source code yet, but the above try is working for me (the TWIG templates recompiled?).
I hope this is useful to you.