The modern festival is a compound, and most of its parts can be dated.
All Souls' Day on 2 November, instituted at Cluny around 998, gave medieval Europe an authorised season of praying for the dead, and with it the practice of souling, the poor going door to door offering prayers for the dead in exchange for soul cakes. That is the ancestor of trick-or-treating, and it is a Christian custom.
Guising, children in costume performing for food or money, is documented in Scotland from the late nineteenth century, and Scottish and Irish emigration carried both practices to North America.
Trick-or-treating under that name is a twentieth-century North American development, first attested in print in the 1920s and 1930s and becoming general after the Second World War, from where it has been re-exported to Britain, Ireland and much of the world as an American custom, which it is, and which is a good example of a practice returning to its source unrecognised.
The jack-o'-lantern was a turnip in Ireland and Scotland. The pumpkin is American, being far larger and easier to carve.
The witch, cat and pointed hat iconography is nineteenth- and twentieth-century commercial imagery, developed in greeting cards and later in film, and bears no relation to early modern witch trials or to anything Samhain.
Modern Pagan and Wiccan Samhain, one of the eight sabbats, dates in its present form to the mid twentieth century and draws on the Irish material, on Frazer, and on the reconstruction of a Wheel of the Year that no single ancient culture kept in that form.