A reason Nick Drake's appeal reaches further than the usual cultish following for British folk-rock singer-songwriters of the late 1960s and the early 1970s is that, though on the surface he appears to share many traits with others in the genre, his albums also contain some quirky surprises that others didn't explore. An underrated facet of the Bryter Layter album's appeal is the use of three beautiful instrumental pieces to break up the more conventional singer-songwriter outings with lyrics. The concluding instrumental, "Sunday," could be the most glorious of those, combining folk-rock with jazz, classical, and even some possible soundtrack influences. Like so much of Drake's work, it projects a fetching melancholy that stops short of bleakness, due to a captivating melody but also a sense of dignity. Certainly the combination of lilting flute, somber orchestration, and Drake's own accomplished guitar playing combine to set a mood that's both pastoral and wary of troubles around the corner. Too, as in other Drake songs, the main melancholy themes are set off by brighter, cheerier sections that are soothing without losing a tension between a placid surface and darker undercurrents. The soundtrack elements come most to the fore at some of the sunnier intersections. But a bittersweet feeling pervades most of the piece, particularly at the very end, which comes back to the same lonely flute figure that started the track. This ends not just the song but the entire album on a pleasant yet haunting note, like a lingering, unanswered question mark.
(Richie Unterberger, allmusic.com)