No flower is more evocative of Summer for me, than the Oleander.
It comes in various colours; white, pink, and red, but for my garden it has to be red.
My one (above) is badly planted in a rather shady spot, and has only just started to flower; even so I can't pass it without smiling.
It should be noted that almost every aspect of the Oleander is highly POISONOUS. Take note children!