Tainy produced the song, while he co-wrote it with Balvin, Lipa, Bad Bunny, Alejandro Borrero, Clarence Coffee Jr., Tory Lanez, and Ivanni Rodriguez.

The song reached the top 10 on the charts in various Central American countries, and received multi-platinum certifications in the United States, Mexico, and Spain.

[3] Tainy, J Balvin, Lipa, and Bad Bunny wrote the song alongside Alejandro Borrero, Clarence Coffee Jr., Daystar Peterson, and Ivanni Rodriguez.

[18][19][20][21] Bad Bunny samples Puerto Rican singer Tito El Bambino and Jadiel's song "Sol, Playa y Arena" (2007) during his verse.

[22][23] A bilingual English and Spanish song, Lipa and J Balvin voice two alienated romantic partners as they employ a call and response technique on it.

[16] The lyrics are an expression of being unable to see "a good thing" even when it is right there; Bad Bunny and J Balvin try to court estranged ex-girlfriends by promising vacation trips to Marbella and Turks and Caicos.

[26] Ana Monroy Yglesias, a staff writer for Grammy.com, shared a similar opinion, stating that "Un Día (One Day)" is "a perfect 2020 summer jam", and praising its melancholy nature and "infectious, slowed down reggaetón beat".

She went on to say that it "remains true" to Balvin's roots: "matching his latin culture heritage against house music, future-facing pop, and hip-hop".

[29] Stereogum writer Chris DeVille named "Un Día (One Day)" a "midtempo reggaeton pop-crossover ballad of sorts" and thought it was an "Event Song" he'd expect to hear in "ad campaigns, inspirational montages, and heavy Top 40 radio rotation".

[31] The staff of The Face named the song a "bilingual banger" while stating that the blend of Latin rhythms and Lipa's "irresistible" pop sound is "a recipe that's destined for success".

[34][35][36] The Recording Industry Association of America (RIAA) certified the song quindecuple Platinum (Latin) in the US, which denotes 900,000 units based on sales and track-equivalent on-demand streams.

[62] "Un Día (One Day)" also placed on numerous best of 2020 year-end lists, including unranked ones by Billboard for best Latin music and Rolling Stone for pop collaborations.

[24] Rose drew a connection between the video's storyline and the song's lyrics; she thought that while "going through life alone", the actress is "processing the [same] breakup that Lipa sings about on the track".