HAPPY BIRTHDAY TO ME was produced by John Dunning and André Link, as a Cinépix production. They famously teamed up again on MY BLOODY VALENTINE (1981), which went into production within a week of this wrapping. However, the latter film was actually released first, rushed to meet a February 11 1981 release date in time for that year's Valentine's Day. Keen to get their slice of the slasher movie pie (or in this case birthday cake), they quickly realised that dates in the calendar were being used up. They figured, quite reasonably, that everyone has a birthday, so it could have universal appeal. They hired John Saxton, a University of Toronto English professor, to develop the story. Dunning claims the idea for the brain injury section of the movie came from him reading an article where they were regenerating frogs with electricity (in fact, this became a scene within the film). He figured this could form the basis for a murder mystery where a girl suffers flashbacks and blackouts, and is unsure of her role in the mayhem around her.
Although it seems to have been directly influenced by the success of FRIDAY THE 13TH, it is worth noting that pre-production on the film had started before that film had been released, which more than hints that the slow burn success of HALLOWEEN (1978) was perhaps more of an influence (although the Grand Guignol elements of Cunningham's film may have convinced the makers of HAPPY BIRTHDAY TO ME to 'beef' up the killings as production geared up).
The excellent Retro Slashers website has a downloadble script purporting to be a third draft from April 1980 (it can be found at http://www.retroslashers.net/happybirthdaytome.pdf). The major difference in this version of the script is that Virginia is actually the killer – albeit seemingly possessed by the spirit of her mother (she speaks in her voice at the very end asking her father what size slice of cake he would like before moving to slit his throat as the action fades to black). Although this makes more sense than the equally loved and loathed end that was filmed, it is easy to see why the filmmakers might have thought that what was originally scripted was not climactic enough (although the majority of the film does appear to have been pointing to this original ending, which indicates the switch came well into production). This version of the script also features a good number of scenes that were either never shot or rewritten, including some that show more clearly Alfred's unrequited love for Virginia, and Virginia's difficult relationship with her father.
The fledgling script was completely reworked by screenwriting team Timothy Bond and Peter Jobin before production started.