Use a column head as a value for a new column

Hello!

Now in my data set I have a column to enumerate the weeks (in portuguese week = semana) and a column for each day of the week (in portuguese: segunda = monday, terça = tuesday, quarta = wednesday). How can I tranform the dataframe above in just three columns with "semana" (the column I already have), day of week and the float value?:

I did some ilustration to explain what do I want:

Please, I appreciate any help.
Thanks, regards from BR.