Why Holikadahana is celebrated one day before Full Moon Day of Chaitra or the advent of spring.
Ritu is important for procreation. It is also related to fertility cycle. Spring is the best period for fertility. It is marked by a change over from cold to warm season. It is also marked by the large varieties of flowers and leaves that spring up to life. People contact cold during this change over period. Hence, the previous night, people gather around fire to give up the last vestige of cold. The next day, then apply the dried and powdered leaves and flowers to their body and take a thorough bath, which cleanses the bodies. This natural principle is enforced by making it ritualistic.
Read science behind holika dahan
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