There is nothing special about seeing flowers on sale around market in Split, but these days there is a difference. Not only market, and not only in Split; streets of Dalmatian and Croatian cities are flooded with chrysanthemums. It is usual picture in days before November, 1, All Saints' Day, or Day of the Dead in local tradition.
On this occasion most of the people will pay tribute to their beloved ones who passed away, to lay flowers and remember them. Cemeteries will be glowing with candles, creating an image of dignity, almost sanctity. It might not be as picturesque as the Mexican Dia de Muertos, but Dalmatian customs are still impressive, full of respect for ancestors. Flowers and candles are usual part of this day, even though some theologists repeatedly warn that over-spending is far from the spirit of this day. And why chrysanthemums? It's hard to tell, but it's a fact that it's really rare to see them selling on any other part of the year. In some other parts of the world, it's usual to bring those beautiful flowers to a host when visiting someone. Don't do it in Dalmatia; those big white and yellow flowers are mostly reserved for cemeteries.
Also, although you will see some Halloween (in Croatian it's Noć vještica - Night of the Witches) decorations on sale in Croatian stores, and even parties in clubs and bars (check out events calendar), these two festivities are not to be mixed. It's not unusual that influential Catholic Church dignitaries would call their parishioners to restrain from this "unholy" event. It may sound silly to give so much attention to something as innocent, but local customs are local customs, aren't they? Of course, no need to worry someone will try to lynch you for dressing up or carving a pumpkin (local expat community is getting ready for this very carefully). My daughters always loved to bring some friends over at our place to carve pumpkin and have a cake-eating with masks. However, Europe has its own carnival time on Fat Tuesday, and trick or treating is something almost nobody practices.
Of course, All Saints' customs are not completely spiritual, there are some more Earthly senses to be satisfied. It's considered proper if a man gives a woman (or bring it home) some bobići (pron. bobeechy), wonderful brown and white sweets made of almond and cocoa. As many other food habits in Dalmatia, this one probably arrived from Italy, since in the area around Trieste there is almost the same treat called fave dei santi or fave dei morti. And you don't even have to be a culinary grandmaster to make them, bobići are available in every pastry shop, even in supermarkets.