Friday the 13thBMJ 1994; 308 doi: https://doi.org/10.1136/bmj.308.6939.1304b (Published 14 May 1994) Cite this as: BMJ 1994;308:1304
- S D Walter
- McMaster University Health Sciences Centre, Hamilton, Ontario, Canada L8N 3Z5.
EDITOR, - C D O'Brien suggests that the 13th of a month is more likely to be a Friday than any other day,1 which has possible implications for the risk of transport accidents.2 O'Brien's calculations assume that the Gregorian calendar in current use has a cycle of exactly 400 years. This assumption is incorrect. …