The origins of Halloween go back to the pre-Christian Celtic calendar. Samhain, as it was then known, was the beginning of the new year--a natural choice given that the air was blowing cold and the ground turning hard. The veil that separated the living and the dead was removed. If you want to combine that Celtic tradition with a more modern point of view this year, what better way than at a concert featuring the excellent Scottish Chamber Orchestra conducted by composer Peter Maxwell Davies. Davies, finally making his Chicago debut this week, is a composer of a large body of fascinating contemporary theater pieces who has recently turned to more symphonic music. Both styles will be evident in this performance, which features two provocative works based on Maxwell Davies's fascination with his Scottish retreat on the remote isle of Orkney, where many of the old folk customs still exist: Into the Labyrinth, a cantata for voice and orchestra featuring tenor Neil Mackie, and An Orkney Wedding, With Sunrise. Rounding out the program will be two popular pieces by Mozart: Symphony no. 35 (K. 385, the Haffner) and Piano Concerto no. 20 (K. 466), with soloist Cecile Licad. Monday, 8 PM, Orchestra Hall, 220 S. Michigan; 242-6237.