Rachmaninoff is probably best known for his extravagant piano concertos, but for my money a few of his atmospheric, achingly spiritual orchestral and choral works rank higher in his oeuvre–namely The Bells, the Symphonic Dances, and All-Night Vigil, the last of which makes up the Grant Park Chorus’s entire program at both of its concerts this week. All-Night Vigil, whose 15 sections actually total only about an hour, is so named because it’s modeled on the series of unaccompanied choral pieces that punctuates an all-night ceremony in the Eastern Orthodox Church; the Rachmaninoff version is sometimes referred to as Vespers, after the evening service that begins such a vigil. Though its text depicts the death and resurrection of Christ, the piece can be seen as the aristocratic Russian’s elegy for the motherland. When he composed it, in 1915, he’d only recently returned from abroad after fleeing the political unrest that was soon to erupt into the Bolshevik revolution; in 1917 he would leave again, this time forever. A friend and student of Tchaikovsky, Rachmaninoff was one of the last links to late-19th-century romanticism, but for All-Night Vigil he took inspiration as well from medieval chants–dark, richly harmonic, deliberately paced Eastern Orthodox melodies that had rarely been heard, even in churches, since the 1600s, and that probably only reached his ears in the first place thanks to a revival of old liturgical music that began in the early 19th century. The work, which Rachmaninoff once told a New York Times reporter was one of his two best, displays his skill at blending voices and, most impressively, writing for basses: its deep resonance and weighty sonorities have an eerie, almost mystical beauty and melancholy. Few stateside choirs have tackled it, no doubt in part because of its Russian text. But despite high turnover, the Grant Park Chorus has consistently excelled in the Russian repertoire–it delivered a wonderfully sung Boris Godunov in the early 90s, for example–and the only place in town to find a better bass section is the Chicago Symphony Orchestra Chorus. Currently without a leader, the Grant Parkers have been trying out various candidates; here the conductor is Twin Cities veteran Dale Warland, who’s led the Dale Warland Singers for almost three decades. The featured soloist is tenor William Watson, a regular with Music of the Baroque and the Lyric Opera. Wednesday and Thursday, 7:30 PM, Holy Family Church, 1080 W. Roosevelt; 312-742-4763. TED SHEN