Spring, summer and autumn can all be good and winter occasionally amazing. There is huge variation from year to year, day to day, hour to hour. It also depends on the style of climbing you have in mind, if you're flexible about this it's rare to have a week go by without some chance of climbing outside.
High summer is better for Limestone sport climbing whereas the gritstone edges can be prone to midges and the friction is not ideal for hard bouldering. If you're not focused on ultimate performance and don't choose a sheltered spot the grit is still fine.
Spring and autumn can be good for both grit and limestone but some of the better sport climbing may suffer from seepage.
Winter has intermittent cold and crisp days which are fantastic for gritstone bouldering and this is typically when the hardest ascents get done but daylight is in short supply and the weather can be miserable for weeks at a time. A risky time to visit.
"It also depends on the style of climbing you have in mind, if you're flexible about this it's rare to have a week go by without some chance of climbing outside."
That’s kind of the response I expected, and being a New England climber, adversity is always part of the recipe up here. I’d honestly just like to climb as many classic trad / sport routes as possible when I visit. I was thinking about an April trip.