How can i get the name of the past month?

Months are admittedly a little tricky to deal with using strictly OOB functionality.

If you have the Date and Time Utilities plug-in installed (which there's no reason not to, IMHO), you could utilize the "subtractmonths()" function.

text(subtractmonths(now(), 1), "MMMM")

Edit: Ujjwal gives a good solution below , using the OOB function "eomonth()".  An additional OOB-only approach would be to just use the month() function to get the month # you want, assemble a fake "date value" using that, and use that to produce the text() output desired:
(edit2: this works fine for months other than January, so a manual check would need to be added to check for "0", though if anyone has a code-minimal trick for this then i'm curious!  i guess it's a good reason to use "subtractMonths()" or "eoMonth()" though.)

Months are admittedly a little tricky to deal with using strictly OOB functionality.

If you have the Date and Time Utilities plug-in installed (which there's no reason not to, IMHO), you could utilize the "subtractmonths()" function.

text(subtractmonths(now(), 1), "MMMM")

Edit: Ujjwal gives a good solution below , using the OOB function "eomonth()".  An additional OOB-only approach would be to just use the month() function to get the month # you want, assemble a fake "date value" using that, and use that to produce the text() output desired:
(edit2: this works fine for months other than January, so a manual check would need to be added to check for "0", though if anyone has a code-minimal trick for this then i'm curious!  i guess it's a good reason to use "subtractMonths()" or "eoMonth()" though.)

Thanks a lot!