The ERA-Interim dataset is made available for download with an offset of about three months from the dataset's reference date. So January data are available towards the beginning of April, February data are available in May and so on.
For the latest available date please see the ECMWF data server page for ERA-Interim. There is no automatic way of checking the latest available date.
The delay is due to two factors: First, the release is delayed for two to three months to allow for production of the latest data, quality assurance and potential technical issues. Second, since data is released monthly, a dataset produced just after a release date will only be included in the next month's release. These add up to an overall delay of about three months.
For more information on ERA-Interim please see the ERA-Interim page at ECMWF.