Despite having success as junior, Day has mostly spent time playing on the ITF Women's Circuit instead of WTA Tour, due to her ranking.

[5][6] She also achieved her best doubles result at a major event as a runner-up at the 2016 Junior US Open with partner Caroline Dolehide.

Day made her WTA Tour debut at the Connecticut Open in New Haven, after reaching the main draw as a lucky loser, having defeated Naomi Broady and Kirsten Flipkens along the way.

After almost five years of absence at the WTA 1000 level, she qualified for the main draw at the Guadalajara Open where she lost in the first round to Eugenie Bouchard.

[15] She claimed the title at the Championnats Banque Nationale de Granby, defeating Katherine Sebov in the final and moving to world No.

[17] At the Indian Wells Open, she entered the main draw as a lucky loser replacing fourth seed and defending champion, Elena Rybakina, directly in the second round[18] where she lost to Nadia Podoroska.