Unlike the Roman system of naming months, prevalent in much of Europe, and the English-speaking world; in Finland the names of the months are based on farming and nature.
The Finnish word for moon (or month) - kuu - is appended to a descriptive word to give the month name.
January (Tammikuu) - Tammi is a dam or weir. January divides the winter into its two halves, as a dam divides a river. So "Dam Month".
February (Helmikuu) - In February, when the sun returns from its winter nap, the crust on the snow, and the icicles shimmer like pearls (helmi); so February is "Pearl Month".
March (Maaliskuu) - The winter snow begins to melt in March, and the earth (maa) becomes visible again, hence March is "Earth Month".
April (Huhtikuu) - Early spring is the time for chopping down pine trees. Huhtapuu is an old name for kaskipuu, or pine tree (literally a tree (puu) grown on land cleared by burning). So "Pine Month".
May (Toukokuu) - The long winter is over, and if you want any crops this year you had better start sowing (touko); hence "Sowing Month".
June (Kesäkuu) - kesä was originally the word plough, later becoming used for summer; thus "Ploughing Month".
July (Heinäkuu) - Time for making hay (heinä); hence "Hay Month".
August (Elokuu) - August is the month of the harvest (elonkorjuu). However, elonkorjuukuu would be too much of a mouthful, so just crop (elo) is used; giving "Crop Month".
September (Syyskuu) - Another simple one, syys is a prefix form of syksy (autumn); so "Autumn Month".
October (Lokakuu) - With the autumn rain comes mud (loka), covering the roads and farm yards; hence "Mud Month".
November (Marraskuu) - Marras is an archaic word for death. After the summer and autumn, the world turns dark, and the year dies; thus "Dead Month".
December (Joulukuu) - Another simple one, December is the month of Yule (joulu).
Note that when giving a specific date, the month should be in the partitive case, so January 5th is 5. (viides) tammikuuta
Loosely translated from http://www.koulukanava.fi/historia/ajanlasku/nimet.htm
A little assistance from gn0sis and email@example.com