There available solutions are:
•Use a simple font to display 23/30 and 24/31 fractions in 3 styles that roughly corresponds to the general typographic families of serifed, sans-serif and fixed-space.
•Use XPress Tags to fully specify the manner in which each element of the 23/30 and 24/31 fractions. This option is not supported by InDesign.
•Use native formatting to specify the formatting of each individual part of 23/30 and 24/31 fractions (namely; 23, 24, 30, 31, and the separator / whose text can be overridden).
There are also some ways of dealing with the issue of displaying months requiring 6 weeks in a layout that only has 5 week rows, without the use of 23/30 and 24/31 fractions.
•The wrap to top option moves days 30 and 31 of the 6th week to the top-left of the minicalendar.
•You can also use the display 1 and 2 on top option to move days 1 and 2 above the first row (this setting is quite popular in Spain).
These various options, and their advantages and disadvantages, are discussed in the corresponding topic on minicalendar options.
Topic 185110, last updated on 16-Jan-2021