Try these suggestions:
Using Ribbons, Florist Wire Or Fishing Line to Hang Wreaths on Windows
Put a piece of ribbon, florist wire or fishing line through the top of the wreath forming a loop and tie a knot at the end. The length of the ribbon will depend on your windows and at what length you want the wreaths to hang.
Now lower the top window sash enough to slip your wreath out. A nail or a screw cup if you want a nail, in the top edge of your interior window casing will hold your ribbon or wire loop. You could also loop the ribbon around your window latch and then shut it close. The ribbon or wire gets hidden from inside by curtains and wire and fishing line do not show outside.
Using Hooks
Command Hooks or nails on the top of your windows are an easy option to hang wreaths from. In fact, nails driven into walls once, will make hanging wreaths a breeze every year. Paint the nails the color of your walls to hide them. Then, hang the wreaths using fishing line or wire.
Suction cups are the key to success for glass windows! Always use suction cups to help support the wreath. One suction cup per wreath is usually enough but depending upon the quality you get, upto four suction cups will help divide the weight of the wreath and also prevent the wreath from getting blown away or banging your windows at night.