Mikulov wine region and lednice castle?

Is it worth it? I understand you are in Ostrava. Comparing to that it is certainly worth it. Especially if you like white wine.
How many days? Well, you want to rent a car tomorrow (Friday) in Ostrava and return Sunday evening in Prague so you got one full day plus two half days. It reminds me when a visitor is asking park ranger in Yosemite (or Yellowstone I am not sure): "What would you do if you got one day to see the park?" And the ranger answers: "I would cry." I think you are in similar situation. Car would be good if you have time to stop in interesting places on your way and did your homework what to see on your way. Besides the route between Ostrava and Breclav is not particularly scenic. (Breclav is the county town for Mikulov, Valtice, Lednice and important railroad crossing).
So here is my suggestion: Go by train from Ostrava to Breclav. You will save time and money. One train is leaving Ostrava hlavni nadrazi (Main Train Station) at 7:00 am arriving in Breclav at 8:48 am. There is no way you could make it that fast by car. If 7 am is too early then another one leaves the same station at 11 am arriving to Breclav at 12:48 pm. There are more trains but I think you should start in the morning. Rent a car in Breclav, drive short distance to Valtice, Lednice, Mikulov, Pavlov. You can do some hiking in Palava Hills - very beautiful, with a little imagination you can think you are in Toscany. You can also rent a bicycle in Valtice and pedal to Lednice, it's all in huge UNESCO park. In Lednice see chateau, tropical greenhouse, minaret. You can do a boat ride from the chateau to minaret. In Valtice there is a Wine Salon of the Czech Republic where you can taste the best 100 samples of Czech wines. Advice: don't taste them all. Although wines from C.R. are not known around the world (they drink it all in C.R.) especially whites are one of the best. Try Rynsky ryzlink, Tramin, Veltlinske zelene. Sauvignon is not bad either. Actually no wine is bad there. Check this website:
http://www.salonvin.cz . You can switch it to English.
There is an interesting Museum of the Iron Curtain in Valtice. http://www.muzeumopony.cz
You would get a lot of information in the chapter Mikulov Wine Region in Rick Steves book: Prague & the Czech Republic. But if you don't have it with you I doubt you could get it in Ostrava but ask. Other source of information - google. And more source local visitor centers and tourist information. On Sunday evening hop on train in Breclav and in three hours you are in Prague. Hard to achieve that time by car (traffic jams especially around Brno and Prague and especially on Fridays and Sundays). It is also possible to do Mikulov Wine Region by public transport but it would be slower. And caution: there is zero tolerance of alcohol behind the wheel in C.R.