*Applauds the turnips, but you have to carve them*
Yes, The Professor is quite aware of Halloween also known as Allhalloween, All Hallows' Eve, or All Saints' Eve are October 31st. It's also his birthday. All Saints' Day (All Hallows') is the next day on November 1st. Samhain runs from sunset on October 31st to sunset on November 1st.
Many important events in Irish mythology happen or begin on Samhain. The invasion of Ulster that makes up the main action of the Táin Bó Cúailnge (Cattle Raid of Cooley) begins on Samhain. As cattle-raiding typically was a summer activity, the invasion during this off-season surprised the Ulstermen. The Second Battle of Maighe Tuireadh also begins on Samhain. The Morrígan (Morríghan) and The Dagda (Daghdha) meet and have sex before the battle against the Fomorians; in this way the Morrígan acts as a sovereignty figure and gives the victory to the Dagda's people, the Tuatha Dé Danann.
The veil between this world and the spirit realm is supposed to be most thin during this time.
The Professor plans on bringing candy also, but maybe not turnips. Typically offerings of food and drink have traditionally been left for the aos sí. If he understands best from The Dresden Files, Pizza seems to be quite popular now-a-days for such purposes.