First Day of Stay?

Considering buying a package from Universal. Confused about “first day of stay”. If I check in late in the day on a Sunday, do I have to include Sunday as one of my park days?

Don’t buy a package. There are no discounts or benefits from doing so. (Unless its the Saphire Falls pre opening discounted packageor special military offer) You are cheaper booking room only either directly with uni or via Orbitz. Tickets are cheaper via 3rd party sites and you can use them any days within a 14 day period. TS dining plan is very poor you can’t use it in any of the hotels.
To answer your question I don’t know the answer should be in the small print.
With room only you only pay 1night deposit and have a 7 day before arrival cancellation clause for refund.

Agreed. Don’t buy a package. Buy everything separate - then you have more flexibility and it can be cheaper. There is no issue in adding the “express pass” as well - they give you a separate “I’m so special” ticket that you take with you to the parks.