They might well celebrate these dates on Andoria and Tellar. But this is Earth's first contact we're discussing, and they're celebrating it on Earth. If you have a Google calendar, you'll note that it will list certain national holidays, then indicate the country, e.g. Independence Day (USA), Victoria Day (Canada), etc... Maybe this one said "First Contact Day (Earth)" on everyone's subspace GooglAmazoDisney Calendar.
Yep. It was a San Francisco school, and the human dad was the one mentioning missing First Contact Day this year. Kima and her mother didn't bring it up. And they went to school anyway, so it doesn't seem to be that big of a holiday