I don't know if there's any way to retrieve the index/subscript directly. You'll probably have to count and increment it yourself:

set -A week_array Sunday Monday Tuesday Wednesday Thursday Friday Saturday
integer idx=0

for day in ${week_array[*]}
 do
  print $idx $day
  idx=$idx+1
done

week_array[0]=Sunday
week_array[1]=Monday
week_array[2]=Tuesday
week_array[3]=Wednesday
week_array[4]=Thursday
week_array[5]=Friday
week_array[6]=Saturday