Sumday was released on May 13, 2003 by record label V2. The album gave the band its highest chart placing in the UK, peaking at No. 22. By 2006, the album had sold 110,000 copies.
An expanded version of the album was released five months later, with a bonus disc of songs recorded live at the Glastonbury Festival in 2003 (tracks 1–6) and three tracks taken from The Black Sessions in Paris (tracks 7–9).
The album was well-received by critics. PopMatters viewed the album as one where Lytle had decided to "tone down on the experimentation, and concentrate on developing some terrific melodies", calling it "really the next logical step for the band". He commented on similarities to the Alan Parsons Project ("Now It's On"), ELO ("The Go in the Go-for-It") and John Lennon ("Lost on Yer Merry Way"), and noted an improvement in Lytle's songwriting. The CMJ New Music Report made it their 'essential release' in May 2003, calling it a "genuinely wholehearted work", and in their end-of-year review placed it at number 7 in their list of the top albums of the year. Jim DeRogatis, in his book Turn on Your Mind: Four Decades of Great Psychedelic Rock, viewed the melodies as "even more effervescent and more memorable" than on The Sophtware Slump. Sophie Best of The Age called it "another sprawling sonic Grandaddy adventure, shimmering with wistful sincerity and rural-tinged psychedelia".NME gave it 8/10, saying the songs sound "pretty much like Neil Young if he'd heard an Aphex Twin record".
Heather Phares of AllMusic, however, criticised the album, calling it "bland and complacent" and opining that it failed to live up to the expectations of The Sophtware Slump.