0 and 7 both stand for Sunday, you can use the one you want, so writing 0-6 or 1-7 has the same result.
Also, as suggested by @Henrik, it is possible to replace numbers by shortened name of days, such as MON, THU, etc:
0 - Sun Sunday
1 - Mon Monday
2 - Tue Tuesday
3 - Wed Wednesday
4 - Thu Thursday
5 - Fri Friday
6 - Sat Saturday
7 - Sun Sunday
Graphically:
+---------- minute (0 - 59)
¦ +-------- hour (0 - 23)
¦ ¦ +------ day of month (1 - 31)
¦ ¦ ¦ +---- month (1 - 12)
¦ ¦ ¦ ¦ +-- day of week (0 - 6 => Sunday - Saturday, or
¦ ¦ ¦ ¦ ¦ 1 - 7 => Monday - Sunday)
? ? ? ? ?
* * * * * command to be executed
Finally, if you want to specify day by day, you can separate days with commas, for example SUN,MON,THU will exectute the command only on sundays, mondays on thursdays.
You can read further details in Wikipedia's article about Cron.