For those who could read, lists of ‘evil days’ were printed in almanacs. On these days people would refrain from travelling as it was believed that those who journeyed on evil days would either never return or would become ill.
There were three Mondays in the year when new ventures should never be started and these were the first Monday in April (traditionally the anniversary of Cain’s murder of Abel), the second Monday in August and the last Monday in December. These latter dates can be linked to the destruction of Sodom and Gomorrah and the birthday of Judas Iscariot. The Christian Church attempted to eradicate the idea of evil days by dedicating each day of the year to one or more saints but up until the nineteenth century superstitions regarding these Mondays as being ‘evil’ continued.
Reference: Medieval England and Superstitions