Opposite of "Mirth":

• Opposite of "Mirth": Sorrow.

• Mirth (Noun)
- Bangla Meaning: আমোদ-আহ্লাদ; আনন্দোচ্ছলতা; হাস্যকলরব; আনন্দোচ্ছ্বাস।
- English Meaning: gladness or gaiety as shown by or accompanied with laughter.

খ) Sorrow (Noun)
- Bangla Meaning: দুঃখ বা বিষাদ; দুঃখ বা বিষাদের কারণ; অনুশোচনা।
- English Meaning: deep distress, sadness, or regret especially for the loss of someone or something loved.

• Other options:

ক) Happiness
- Bangla Meaning: সুখ বা খুশীর অনুভূতি।
- English Meaning: a state of well-being and contentment: joy.

গ) Laughter
- Bangla Meaning: হাস্য; সশব্দ হাসি।
- English Meaning:a sound of or as if of laughing.

ঘ) Celebration
- Bangla Meaning: উদযাপন।
- English Meaning: a special social event, such as a party, when you celebrate something.

Source: Merriam-Webster Dictionary, Accessible Dictionary, Cambridge Dictionary.