Is this a problem with the Calendar display? I don’t see that you have a license for the calendar extension, so not sure why you are posting this in the calendar extension support forum. As stated at the top of every support forum here in a bold sticky post, the support for the main free plugin is at the wordpress.org site for it, so that I can keep all support topics for it in one place: https://wordpress.org/support/plugin/pta-volunteer-sign-up-sheets/
As far as the day not showing, please check the date format in your WordPress settings (not my plugin settings). You set the date format on the Settings > General page in your WordPress dashboard. My plugin will always display the date in whatever format you have set there. So, if you have a format set that only shows the month and the year, then that’s all you see anywhere my plugin displays a date.