The DOE seriously got its act together and posted all open house info for specialized high schools - this usually doesn’t happen until September:
Bronx Science has a sign up component, but it’s not listed at the school’s website and that one is more for head count purposes. They don’t check you in when you get there. LaGuardia also asks you to sign up - they cap their event. Click here to register:
Here are all open house dates. SHSAT and LaG audition dates are below.