Answer:
Once!
Step-by-step explanation:
The bookstore had a sale on any day with a factor of 5.
For the shoe store starting day 3, create a arithmetic sequence up to day 31, with a common difference of 6:
3, 9, 15, 21, 27, 30
Only 15 has a factor of 5! Therefore the stores only share one sale day on the 15th of July.