The months and an explanation of what "end stress" means

[kutsuwamushi]

Many words for months are end-stressed. This means that when an ending is added, the stress shifts to that ending. For example,

nom. декабрь -> gen. декабря

This is true all of the case endings. Once upon a time, the nominative/accusative had a vocalic ending, but this was lost through sound change, and the stress had to attach itself to the next available syllable.

январь January (end-stressed)
февраль Februrary (end-stressed)
март March
апрель April
май May
июнь June
июль July
август August
сентябрь September (end-stressed)
октябрь October (end-stressed)
ноябрь November (end-stressed)
декабрь December (end-stressed)