It was an Autumn, end of the year event until 1979, when it became a late summer event with 1 or 2 events following it. It stayed that way until 2006, when it became a late winter, early spring event to begin the season.
The reason for the first change was to open up more lakes as potential sites, the second change was to get back to it being the final event for a season (even though it was held in different calendar year). They took a cue from NASCAR, who kicks off their season with the Championship from the previous season. It made more sense & gave them time to prepare months for their Super Bowl instead of just a couple of weeks following the last event.
However, after a rash of very cold Classics, including Hartwell in 2015 where boats were frozen to their trailers & couldn't be launched, in 2017 BASS pushed the Classic to mid March in hopes of avoiding ice fishing tourneys. The problem with that is it is a bit too late to then start the Elite series in April, so they decided to hold an event or two before the Classic so as to not cram the entire season into a compressed schedule. Unfortunately, they are now back to where the Classic neither ends nor kicks off the season.
Interestingly enough, the last two Classics have been very compelling with weather changes causing major shakeups in the standings every day. The last 1st day leader to win was Cliff Pace in 2013 at Grand Lake. Further, NO day 1 leaders NOR day 2 leaders for the last five Classics have ended up winning, it has always been someone from down in the pack.