This is the 133rd most frequent Portuguese word.
segunda
"Segunda" in English means "Monday" or "second" depending on the context.
In this sentence, 'segundo' is used as an ordinal number, meaning 'second', and indicates sequence or order, in this case, of the books read.
É o segundo livro que leio este mês.
It is the second book I read this month.
In this sentence, 'segunda' is part of the compound noun 'segunda-feira', meaning 'Monday', which is the second day of the week in Portuguese tradition.
Hoje é segunda-feira, o começo da semana.
Today is Monday, the start of the week.
Here, 'segundo' functions as a preposition or introductory phrase meaning 'according to', conveying the source of the statement.