Can I bunch all my research sessions onto the days my parents have the baby?

Going back to work in about six weeks. My parents have offered three fixed days a week, Monday, Wednesday and Thursday, and they've been very clear that it's three, not "three and sometimes four". My job is mostly user research sessions, and those get booked around when participants can come, not when I can. My manager says I can ask the recruiter to schedule only on my days. On paper that works. In practice I've seen how often a participant cancels and the only reschedule slot is a Tuesday evening. Has anyone with a client- or participant-driven calendar managed to keep it to fixed days for the first year? Or does it fall apart by month two and I should line up a backup for the other days now? (My partner's lab hours are not flexible, before anyone suggests it, ha.)