Marvel comics cover dated April 1980 featured a new subscription form. Marvel had 33 comics in its line up available for subscription at this point, pretty much the full publishing roster with the exception of its magazines and special editions. These subscription ads have proven to be very useful for tracking the months that comics were actually released since the cover dates are typically at least three months ahead. This ad is dated January 1980, which is a pretty good indication of when the April 1980 comics would have been on newsstands. Click on the image below for a high-resolution copy of the ad.
Source: Kraalo Archives