Mackenzie Phillips (Julie) and Valerie Bertinelli (Barbara) were actually the same age. Phillips was taller and wiry, while Bertinelli was round-faced and looked like the younger of the two, so Phillips was cast as the older sister.
The set used as the travel agency where Barbara and Max work during the final season was later used as Al Bundy's shoe store on 'Married With Children (1988)'. In a few episodes of this show, a "New Market Mall" sign is visible through the windows.
According to the book "Archie and Edith, Mike and Gloria: The tumultuous history of All In The Family", the episode All in the Family: Edith's 50th Birthday (1977), where Edith was held prisoner in her home and tormented by a would-be rapist, was originally intended to be an episode of One Day At A Time, with Ann Romano as the victim.
The premise for this show (a family led by a single working mom) was inspired by actress/writer/single mom Whitney Blake. One of Blake's children, Meredith Baxter, would go on to play a sitcom mom on Family Ties (1982).
During the 1979-1980 season, Mackenzie Phillips was dealing with substance abuse and erratic behavior. Her character, Julie, married Max and moved to Houston. In real life, Phillips was given a six-week vacation to go to rehab. In fall 1981, Phillips returned as a guest in a two part episode when Julie and Max have marital troubles. In the final season, Phillips collapsed on set. Associate Producer Patricia Fass Palmer woke her and asked Phillips for a urine test, and Phillips told Palmer not to bother. Phillips was quietly fired again and written out of the series.
Lyrics for the show's theme song: This is it; this is it. / This is life, the one you get, / so go and have a ball! / This is it; this is it, / straight ahead, and rest assured, / you can't be sure at all. / So, while you're here, enjoy the view; / keep on doing what you do. / Hold on tight; we'll muddle through, / one day at a time, one day at a time! / So, up on your feet; up on your feet; / somewhere there's music playing. / Don't you worry none, / just take it like it comes, / one day at a time, one day at a time, / one day at a time, one day at a time, / one day at a time, one day at a time, / one day at a time!
When Mackenzie Phillips left the show the second time around, her character Julie was written out by first going away for training for her job and then having her leave Max and their daughter Annie. Phillips left the series due to documented substance abuse problems.
Bonnie Franklin resented Glenn Scarpelli's performance because his character drew more laughs than hers. Scarpelli left in 1983 to co-star in the short-lived _Jennifer Slept Here (1983)_; his departure was explained by his character's going away to college.