Murphy's Law

I worked eleven days on (which sounds worse than it is - most of my shifts are only three to five hours) and then had five days off. On the afternoon of the fourth day off, after not having been called in to work even once, I invited my in-laws to drive into town and have tea with me on the morning of my fifth day off. Within minutes of my doing so, I received a call from the library asking me if I could work on that same morning. Arrrgh! I had to say no ('bye 'bye, money - sob!) but what can I do? I haven't seen my in-laws in quite awhile and they're not that good with last-minute changes of plan.