New Castle is pleasant to visit year-round, though spring and fall are the most comfortable seasons. Summer weekends bring tour groups and school field trips to the courthouse, so arriving on a weekday gives you more space to appreciate the architecture. The town holds an annual A Day in Old New Castle event in May, when many of the historic houses open their interiors to visitors — it is the best single day to visit if you want to see inside buildings that are normally closed.
Winter is quiet but not unwelcoming. The historic district is at its most photogenic in light snow, and the absence of crowds lets you wander the alleys at your own pace. The waterfront can be cold and windy in winter, but the museums are open.