Having an active subscription means that it is set up to renew automatically. Therefore if you have a prepaid subscription for three or six months, but it is cancelled, you are not eligible for presales even if you are due more boxes. 

We recommend that if you would like to retain presale eligibility, that you do not cancel your subscription until closer to the renewal date. To cancel your subscription, follow this FAQ article.

If you skip a month, as long as your subscription remains active then you are eligible for presales.

Please note that generally, you will only have access to presales based on the subscription you have. For example, an active Afterlight subscription will give you presale access to Afterlight products, and not Illumicrate or Evernight products.