I did quite a bit of reading and this is the general consensus: toddlers need a routine and a schedule to feel secure. The daily timing of meals, naps, and bedtime on that schedule should not vary more than 15 minutes in either direction.
The schedule looks like this:
6:30-6:45am Wake Up, Milk, Cuddle with Mom, Change Diaper, Get Dressed
Walk in Stroller
Play outside or play with mom inside
Run Errands with Mom
5:00pm or 5:30pm Mom prepares dinner, DS watches short video
6:30pm Play outside
7:30pm Bath, Pajamas, Books, Milk
After about a week and a half, DS is back to his easy going self. And, as long as we stick to his schedule most of the time the times when he gets off schedule (Sundays, special outings) are not nearly as hard to overcome as they were before.
Note: I rotate toys so his free play activities vary every day or every other day. I have a basket in the living room that I change out when I rotate toys.