CDs were deregulated in October 1983 and because they were first sold in 6 month increments, many of them came due in October and April of any given year. Bank Rate Monitor once estimated that there were upwards of $100 billion in CDs rolling over in those two months.
To get rid of those two "blips" each year, more banks began offering odd-term CDs, such as seven and fifteen month terms.
The Result: The blips disappeared as more CDs began maturing in odd months.
Currently, the months of January, February and March show as the heaviest rollovers - not October or April.