For an overview of the Old English calendar and links to the other months, see the summary page.
The third month of the year, corresponding approximately to the Roman and modern month of March, was called Hrethmonath. Bede, writing in 725, tells us:
Hrethmonath is named for their goddess Hretha, to whom they sacrificed at this time.
--Bede, On the Reckoning of Time, Chapter 15. Translated by Faith Wallis.
Who was Hretha, and what sort of goddess was she? As far as I know, this is the only surviving mention of her, so the short answer is that we know very little about her. Some inferences can be drawn from the name.
According to the online Old English dictionary, the Old English noun 'hreth' means 'victory, glory'. The parallel with the Roman name of the month, dedicated to Mars the god of victory and war, is obvious.
Kathleen Herbert says the corresponding adjective, hrethe, means 'fierce, cruel, rough', and suggests that Hretha was a war goddess or a valkyrie (Herbert 1994).
March is the last month of winter and is quite capable of bringing destructive storms as well as warm sunshine - as the people picking up the pieces in southern and western England and Wales after Storm Johanna in 2008 could testify. There is a traditional belief that the seas around Britain are especially prone to violent storms in March and September, hence the term "equinoctial gales" (The equinoxes are not really associated with storms, incidentally, but there is a grain of truth in the tradition, as the frequency of high winds rises sharply in late September and declines again around the end of March).
So March might well seem an appropriate month to dedicate to a violent goddess of battle, especially if she was also fickle. Perhaps the sacrifice, whatever it was, hoped to placate her and avert the worst of the equinoctial storms. Perhaps also to ask her favour in warfare for the forthcoming campaigning season.
As usual, not proven, but an interesting possibility nonetheless.
Full-text sources available online are linked in the text.
Bede: The Reckoning of Time. Translated by Faith Wallis. Liverpool University Press, 1999, ISBN 0-85323-693-3.
Herbert, Kathleen. Looking for the Lost Gods of England. Anglo-Saxon Books, 1994. ISBN 1-898281-04-1.