Are local wine stores fully closed on Sri Lankan public holidays?

Yes, as far as local rules go, most wine shops, bars, and even the supermarket alcohol aisles in Sri Lanka are strictly shut by law on Poya (full moon) days, as well as on a few major national and religious holidays.

On those culturally significant dates, the sale of alcohol is not allowed anywhere in the country. The restriction covers restaurants too and regular hotels, though a small number of high-end, luxury tourist resorts may quietly provide drinks to resident guests, for example, through room service, usually without making a show of it. If you want to have a beer or maybe a cocktail during a Poya weekend, you need to buy it the day before, then drink it privately at your place to stay.