Rosh Hashana 5779

Is there a mitzvah to rejoice and have festive meals on Rosh Hashanah?

Why don't we say Hallel on Rosh Hashanah?

It's a day of Judgment, why is rejoicing appropriate?

Is Rosh Hashanah a moed - it's called a moed in the Torah but we don't refer to it as such in davening or kiddush?

The Shulchan Aruch writes that we eat, drink and rejoice on Rosh Hashanah, paraphrasing the Gemara which contrasts how we approach the Day of Judgment and how the gentiles do.