The usual form is "readiness". According to the OED, "readyness" went out some time in the 17th century. I think terminal "y"s quite often turn into "i" when a

The usual form is "readiness". According to the OED, "readyness" went out some time in the 17th century. I think terminal "y"s quite often turn into "i" when a suffix is added (ready -> readier; daily -> dailies; thirty -> thirtieth); there is probably an interesting reason for this but I don't know it.