April 9th, 2019 is...
National Gin & Tonic Day (US)!
Thanks to Stevie Watson of Pickering’s Gin for clarifying that the April G&T day seems to be a US Celebration rather than International.
Folks outside the US are still allowed to celebrate.
Some links of possible interest:
Here's To Your Health! It's International Gin & Tonic Day at LAist
International Gin & Tonic Day: Celebrate With Tasty Cocktail at Hamptons.com
Wikipedia: Gin and tonic