When shooting landscapes, its usually best to get up really early or get out really late. The minutes just before sunrise and just after sunset have what is called "golden light." Makes for very beautiful shots. Between those times, the different light available can be good for certain shots. Before about 10 or after about 4 you get nice direction in light and makes for nice landscapes. Mid day - its best to look for close ups in the shade or go for lunch.
Of course, if there is no sunshine, you can still get some great shots. Colours can be very vivid and saturated on cloudy days - although distance shots and anything with sky in them are generally (not always) not as attractive.
