According to the great and wise Wikipidia,

>This extra day added to the last month was outside of the days of the week cycle, and so the first of a month was always a Monday. On leap years, an additional festival day (also outside the week cycle), to celebrate holy women, would join the memorial day of the dead.

Which to me reads that the extra day/s are not one of the 7 weekdays. Hence these days would fall between a Sunday and Monday, but would not be refereed to by any of our current weekday names. So, each year there would be the forth Sunday of Bichat, followed by the The Festival of All the Dead epagomenal day, followed by the first Monday of Moses.

edited out some typos