Alnwick is at its liveliest between April and October. The castle and garden open for the season in spring, the garden's Grand Cascade runs at full effect in summer, and the Northumberland coast is at its best on warm September days when the summer crowds have thinned. The Alnwick Food Festival in September draws producers from across the region and is worth timing a visit around.
Winter is quiet — the castle closes between November and March, and much of the garden shuts down — but Barter Books stays open year-round and is particularly atmospheric in the dark months, with fires going and fewer visitors competing for the armchairs. The Northumberland countryside is beautiful in late autumn, and the coast at Embleton Bay or Bamburgh, an easy drive north, is magnificent in any season.
Northumberland has some of the least light-polluted skies in England; the Northumberland Dark Sky Park is centred on Kielder Water, about 50 miles northwest. If you are interested in stargazing, October through February, away from full moon, are the best times.