According to the listing (via Nintendo Life), Amazon Spain has the game listed for a release on the 22nd of June. This means that this is more of a summer release than a spring one, although given that this is Amazon Spain, there is a chance that the June date could be for Spain or the Latin America market.
It is also possible that the date may simply be a placeholder date, albeit a very specific one. This is versus Amazon US which has the game listed for the 31st of December, 2018. However a summer release means that the game is “later” than what Nintendo had promised, but we’ll have to wait and see. Mario Tennis Aces is not the first Mario tennis game that Nintendo has launched.
Previously the company had titles like Mario Tennis: Power Tour, but clearly this was designed for the Switch in mind where presumably it will take into account the motion sensors inside of the Joy-Cons. In any case take it with a grain of salt but we’ll keep an eye out for a more official release date in the near future.