Why doesn't code like `"A %s", "B" % "C"` interpolate "C" into the first string?

Im wrtiting a script which saves the current date and time as a filename but I get an error stating "TypeError: not all arguments converted during string formatting" I am new to Python andmay of missed something obvious. Code below:

from subprocess import Popen
import datetime

today = datetime.date.today()

today = str(today)

print today

f = open("%s.sql", "w" % (today))
x =  Popen(["mysqldump", "-u", "root", "-pucsdrv", "normalisationtion"], stdout = f)
x.wait()
f.close()