I let her day-off at different days every week. usually Monday or Tuesday, depending on my activities and the son's summer activities. Usually I give 3-4 days notice. Sometimes I just change it if I have something to do.
One point to note is that I let her sign acceptance to rest on weekdays before I hire her overphone (arranged by agency). So, she has no say in choosing Sundays or Saturdays to rest. Sometimes she asks if whe can rest on Public holidays, and I will promise in some cases, but not all.