Currently my application is using log4net to log errors, the web.config for this is as followed:
You should use RollingFileAppender with rollingStyle Date and datePattern yyyyMMdd (this will roll every day).
Example of configuration:
UPDATE: I think a better way is to include the date into file name. And use datePattern value only to show when new log file should be created.
In this case all your files will have name like gateway_20120427.log.