library(zoo)
m <- factor(months(zoo::as.yearmon(time(datasets::sunspots))),
            levels=month.name)
plot(m, datasets::sunspots, axes=FALSE)
axis(2)
axis(1, at=1:12, labels=FALSE)
library(gridBase)
vps <- gridBase::baseViewports()
pushViewport(vps$inner, vps$figure, vps$plot)

grid.text(month.name, 
          x=unit(1:12, "native"), y=unit(-1, "lines"),
          just="right", rot=60)
popViewport(3)