From "What Does This Fire?" to Read Once, Trust It
Before: someone writes 0 9 1,15 * 1-5 at 11pm. Six months later, three people argue about whether it fires the 1st AND 15th, the 1st OR 15th of weekdays, or the 1st of the month plus weekdays.
After: pick a schedule type by its actual intent — "Month / Specific Days" with the days you want and the days-of-week filter. The setting reads the same in six months as the day you wrote it.