How do I specify a sort order using a list?

You can do it "inline" with

perday[order(factor(perday$dayofweek,levels=c(c("Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun")))),]

which gives

   dayofweek Attended nrow
3        Mon       No  142
4        Mon      Yes  174
11       Tue       No  184
12       Tue      Yes  214
13       Wed       No  343
14       Wed      Yes  255
9        Thu       No  191
10       Thu      Yes  192
1        Fri       No  164
2        Fri      Yes  131
5        Sat       No   39
6        Sat      Yes   26
7        Sun       No   44
8        Sun      Yes   39