Summers are lovely but along with your shorts and beachwear pack a light-weight raincoat - the odd shower cannot be ruled out. Winter winds can be cold and you should certainly wrap up well in January and February. The average temperature in these months hovers just above 35F. One of the nice things about Holland is that the Dutch are relaxed about clothes. You wear what you feel good in - particularly in Amsterdam where an opera audience happily turns up in jeans and t-shirts. Which doesn't mean you cannot dress up. Men should bring a tie and women a skirt or dress if you plan dining up-market. Otherwise feel free.