the average temp @ mt washington in april is 22.9 degrees with a record low of -20 in 1995. definetly use the layering system http://www.rei.com/learn/Camping/rei...ADVICE_CAMPING
REI has a crazy library of free information heres they link that will give you the information to make a perfect clothing set-up.
above all realize that this peak is known for its title having "the worlds worst weather" we all need to respect that and prepare for the worst.
"Auto racing, bull fighting, and mountain climbing are the only real sports ... all others are games." Ernest Hemingway