@Christian-Bartlett-7mpfcnb I feel like the monthly catch-ups are always announced on the first of the month unless it is a holiday or weekend. Then it will fall on the next regular business day. It also is normally announced late in the day (depending on your timezone), which can provide some people with an extra couple hours to finish catching up.
As for what books are on catch-up, they usually play that close to the vest, likely in part to licensing agreements. But the selected series, when not entirely "random", seem to be associated with the start of a new volume being translated, especially if it has been on hiatus, or to take advantage of a new anime season.