Answer:
dear XYZ,
hope you are healthy and your parents also as you know that there was a festival of dashain , hope you enjoyed the festival like every year , sorry that this year I wasn't able to be with you during the festival as I have gone to my grandparents home for the celebration
I was very excited as I was going to meet my grandparents and also going to celebrate the festival , as you know while the festival we are supposed to clean the whole house and have to decorate the house so I helped my grandparents for the decorations and after that the day when the festival was we did the rituals and all the things while for the ceremony , and prayed to bring happiness in the house and after the rituals were given money from the elders and my siblings and after that we gathered and sat on one place for the delicacy that is mutton I litterly love to have mutton cooked by grandma so, we eat out meal after that I went to play with my siblings the remaining day went very well
hope you also enjoyed the festival and also be happy and I was missing you there while I was in my grandma house
your loving
XYZ
if it is big than you can remove some lines from it , hope it helped you :)