Derivation of Days and Months

Days

Sunday - Named after the Sun
Monday - Named after the Moon
Tuesday- Named after Tiu, the Norse god of war
Wednesday - Named after Woden, the Anglo-Saxon chief of the gods
Thursday - Named after Thor, Norse god of thunder
Friday - Named after Frigg, a Norse goddess
Saturday -Named after Saturn, Roman god of harvests

Months

January - Named after Janus, Roman god of doors and gates
February -Named after Februa, Roman period of purification
March -Named after Mars, Roman god of war
April -from the Latin apeire, 'to open'
May - Named after Maia, Roman goddess of spring and growth
June -Named after Juno, Roman goddess of marriage
July - Named after Julias Caesar
August -Named after Augustus, first emperor of Rome
September -from the Latin septem, 'seven'
October - from the Latin octo, 'eight'
November - from the Latin novem, 'nine'
December -from the Latin decem, 'ten',

ENGLISH     ITALIAN    SPANISH    FRENCH
Monday      lunedi     lunes      lundi
Tuesday     martedi    martes     mardi
Wednesday   mercoledi  miércoles  mercredi
Thursday    jovedi     jueves     jeudi
Friday      venerdi    viernes    vendredi
Saturday    sabato     sábado     samedi
Sunday      domenico   domingo    dimanche

Why the decisions were made is still beyond me; why the fifth day of the week belonged to the goddess of love and not the goddess of, say, the harvest, or any other deity, seems to have been made arbitrarily.

It seems fairly safe to say that the Romans named the days first and then, during their conquests and fall, their neighbors and former vassal states took these ideas and made them their own.(4)

Another bit of interest is that in Italian and French, the days belong to the gods (jovedi = day of Jove), while the Spanish just use the possessive (jueves = Jove's).

Now, in German, we have:

Sonntag: like Sunday
Montag: like Monday
Diensdag: "Assembly Day"; possibly from the assembling of soldiers?
Mittwoch: mid-week, literally
Donnerstag: "thunder day"
Freitag: like Friday
Samstag: presumably like samedi

(2) Jove (Jupiter) was the leader of the Roman gods, yes, but one of the major factors of his aspect was as the god of the lightning.

(3) Frigga was the Norse equivalent of Juno. However, the Norse goddess of love (and analogue of Venus) is Freyja; the similarity of names may indicate that the goddesses were combined into one, or that Friday was named after Freyja instead. The similarity could also be meaningless and the parallel false.

(4) This is simply speculation on my part; please don't write a paper on it without backup sources.

It may seem rather strange that some of the names of the months do not match the number of the month. For example, November, the 11th month of the year, means 'nine' and December, the 12th month, means 'ten'. The reason for this is that the Roman calendar originally only had 10 months, beginning with Martius (March). King Numa later increased the number of months to 12 by adding Januarius and Februarius at the end of the year.

When Julius Caesar altered the length of the months in 46 B.C., he also shifted them, resulting in the year beginning with Januarius and ending with Decembris - the original 10th month of the year.