You might try using the last command to see a list of the logins for the past month (with most distributions). The actual log file that is used for the last listing is /var/log/wtmp.
Also, many distributions archive the monthly logs. So if you see files such as /var/log/wtmp.1 /var/log/wtmp.2 and so on, these are typically logs from previous months. To access thes logs you can use the command:
last -f /path/to/file/filename