Consider you can do tastings and winery visits directly at Castello di Banfi too! If you're staying there, make sure to email them ahead of time and ask for their hours/times in case you need to book.
Most places are open on Sundays only once the "season" starts, which is generally around Easter... this year Easter is a little later than usual so I'm thinking wineries will start opening sooner, rather than later. I would suggest heading toward Montalcino for winery visits or exploring the wine area of Montecucco DOC, which is toward Civitella Marittima.