There are a few very obvious similarities between Tihar and Halloween. The first similarity is in the custom called trick or treat. Children dress up in fancy dress costumes, which are usually scary, and visit their neighbors where they play the game of ‘trick or treat’. They ask the neighbors to give them a ‘treat’, which is usually candies and chocolate. If not, they end up playing some minor ‘trick’ on the neighbor, like puncturing their tyres.
In Nepal, we thankfully avoid the trick part, and our costumes are meant to make us look pretty rather than scary, children do go around to their neighbors’ homes and get lots of Tihar goodies in return. Today cash may have replaced the goodies but there was a time when it was more normal to give rice and food to the deusis and bhailis. It is still not uncommon to find selroti, mithai, and fruits among the Deusi and Bhailini’s booty. Incidentally, this tradition of Deusi and Bhaili seems unique to Nepal, I have yet to come across an Indian Diwali celebration where children sing at their neighbors' houses and are given goodies.
The reason for celebrating Halloween is steeped in many layers of mythology. The end of October and beginning of November are the All Saints Day and All Souls Day, which commemorate the dead. As a result, today Halloween is a day of ghosts, devil, and anything that is scary in Christianity. The association of death also exists in Tihar since Yamraj and his army plays a major role in Tihar. However, the representation of death in Tihar is not so macabre as in Halloween.
Also, going back to the pre-Christian roots of Halloween, Halloween is supposed to be descended from the Celtic festival of Samhain, celebrated in Western Europe over 2,000 years ago. This festival was marked by building huge fires in pits. Today, this fire lives on as candles placed inside carved pumpkins.