The jack-o-lantern is only available on Oct. 31, the snowman and holiday tree on Dec. 25. (jack-o-lantern and snowman in scenery panel, tree in foilage panel)
If you want to keep placing them and know one in game day is not going to be long enough to place them where you want, then watch the bottom of the scenery panel when it gets close to the date when they appear. (say for the pumpkin, oct 29) Then when you see all the objects shift, pause the game then find the object you want and you can place as many as you want while the game is paused.