The monthly security release window for Drupal 8 and 7 core will take place on Wednesday, March 16.
This does not mean that a Drupal core security release will necessarily take place on that date for any of the Drupal 8 or 7 branches, only that you should watch for one (and be ready to update your Drupal sites in the event that the Drupal security team decides to make a release).
There will be no bug fix or feature release on this date. The next window for a Drupal core patch (bug fix) release for all branches is Wednesday, April 06. The next scheduled minor (feature) release for Drupal 8 will be on Wednesday, April 20.
Drupal 6 is end-of-life and will not receive further security releases.
For more information on Drupal core release windows, see the documentation on release timing and security releases, and the discussion that led to this policy being implemented.