The second month of the Shire Calendar, corresponding roughly to modern February (though astronomical changes between those ancient times and today mean that the month actually ran between 22 January and 20 February on a modern calendar). Also unlike our modern second month, Solmath had thirty days (as indeed did all the months on the Shire Calendar), and due to the construction of the calendar, it always began and ended on the same days of the week (Tuesday and Wednesday, respectively).
The name 'Solmath' was generally pronounced 'So'math' in the Shire, and hence the name was sometimes written as 'Somath'. It comes from Anglo-Saxon Solmónað, literally meaning 'month of mud', and this same idea is present in the name chosen for the second month by the Dúnedain, who called it Nénimë, 'watery month'.