אוכלים ושותים ושמחים

"We eat, drink, and are merry!"

The Shulhan Aruch explains that one must eat the Rosh Hashana meal with joy, trusting that Hashem will give him a good judgement. This idea is surely valid for the righteous who will merit a great judgment on Yom Hadin. How, however, can the wicked be so sure?

Rav Nachman of Breslov writes that when one does a Misva with joy, "Hashem protects him!" 

Therefore we instruct even the wicked to fulfill the Misva of Simhat Yom Tov with great jubilance as it will provide him with the protection from any bad verdicts!

בעניין שלושה ספרים

The three books opened in Heaven on Rosh Hashanah

The Gemara in Rosh Hashana teaches that the רשעים גמורים (the wicked people) are instantly written and sealed until the book of death, where the בינוניים (medium level people) have their judgment delayed until Yom Kippur. The Rabbis teach that the Shofar causes Hashem to turn from דין (judgement), instant punishment to רחמים (mercy), delaying punishment in order to afford the Jews time to repent.

If that is true, why do the purely wicked get punished instantly?

As we have explained in other places, רשע גמור, a wicked person, has no merits. It therefore stands to reason that he was not present for the Shofar blasts. Therefore, this fellow is still subjugated to מדת הדין, stringency!