To answer this question, we looked at bond yields for government bonds and corporate bonds going back to 1915, providing over 80 years of evidence. Data on Treasury bills only goes back to 1931, so the evidence is more limited. We calculated the percentage change in interest rates from the end of one month to the next, then calculated the average returns for all the Januarys, Februarys and so forth. We also calculated the changes over the past 50 years, to see if patternschanged after World War II, and we calculated the changes over the past 25 years to see if the patterns changed during the volatile 1970s and 1980s when inflation dominated the interest rate landscape. Treasury bills were only analyzed for the past 50 years because the data for Treasury bills does not extend back to 1915, and the excessively low interest rates of the early 1940s, going as low as 0.01% exaggerate the percentage changes. Based upon the evidence of these series, distinct seasonal patterns in interest rates do appear. There have been some changes in these patterns over time, but the tendency has been to strengthen these patterns rather than weaken them. This has occurred because the percentage change in interest rates from one month to the next has increased over time. Moreover, some months, which were neutral in their interest rate changes in the past, have now moved demonstratively toward changing in one direction or the other. Finally, the monthly patterns in interest rates tend to reinforce the patterns in the stock market.