2 answers
Hi Erin,
As you mentioned the object relationship is master-detail, you can create a roll-up summary field to achieve your requirement. Please check the below screenshot on how to select the roll-up summary as per your requirement.
1. Go to Account object
2. Select Roll up summary as field type
3. Give the field name and click on next which takes you to the below screen shot.
4. Make sure the select the object as Account Visits( in your use case) ad Roll up Type as Max of Date visit.