Code:
$ cat file1
item,So,Mo,Tu
aaa,1,1,1
bbb,1,1,4
ccc,1,1,0
ddd,1,1,1
$ cat file2
item,Tu,We
aaa,1,1
bbb,4,0
eee,0,1
fff,0,2
$ join -t, -a1 -a2 -e0 -o0,1.2,1.3,1.4,2.3 file1 file2
item,So,Mo,Tu,We
aaa,1,1,1,1
bbb,1,1,4,0
ccc,1,1,0,0
ddd,1,1,1,0
eee,0,0,0,1
fff,0,0,0,2