Great Festivals of the World

Snow & Ice Festival - Harbin, China - January (This is by far the one you should check out)
Paryushana Parva - Palitana, India - September or so (Jain festival)
Carnevale - Venezia/Venice, Italy - February or so
Kumbh Mela - Allahabad/ Prayagraj, Uttar Pradesh, India - February or so but every 12 years (the world's biggest gathering of people - 120 million in 2019)
Diwali - India - November or so
Lantern Festival - Pingxi, Taiwan - February or so
Whirling Dervishes festival - Konya, Turkey - November
Carnaval - Rio de Janeiro, Brazil - March or so
Kite Festival - Weifang China - Spring
The Karakuri Marionettes Of Takayama - April and Autumn
Songkran Water Festival - Chiang Mai, Thailand - April or so
Kumbh Mela - Allahabad, India - February
Street painters ("madonnari") festival - Grazie di Curtatone, Italy - May
Pera's sand sculpture festival - Portugal - May
Vesak - Buddhist countries - May
Patios Festival - Cordoba - May
Mardi Gras - New Orleans, USA - March or so
Ratha Yatra - Puri, India - July
Torch Festival - China - August
Shoton Festival - Tibet - August
Balloon Festival - Albuquerque, USA - October
Dragon Boat Festival - China - June or so
Cascamorras - Baza, Spain - September
Oktoberfest - Munich, Germany - October
Ashura - Karbala, Iraq - November or so
Battle of the Oranges - Ivrea, Italy - February or so
Chinese New Year (Spring) - China - January or February
Dia de los Muertos - Mexico - November
La Tomatina - Bunol, Spain - August
Up Helly Aa Fire Festival - Lerwick, Scotland - January
Marrakesh Popular Arts Festival - Marrakesh, Morocco - July
Dragon Boat Carnival - Hong Kong, China - June
Holi - India - March or so (my least favorite of the big ones, but most people go crazy for it, especially in the West)
Glastonbury festival - Britain - June
Pongal festival of kolams - South India - mid January
I doubt you'll live to see it again, but the Maha Pushkaralu festival takes place every 144 years in several locations in India (it happened last time in 2015)

Some of the very famous festivals are missing because i don't want to support them (e.g., the barbaric running of the bulls in Pamplona, the Palio in Italy and Halloween in the USA).