In context, it could make sense. I don't know if it was talking about internet traffic or traffic on the roads, but either would be possible.
Let's say it was internet traffic. Someone has a website and has collected the data on how many people visited the site on any given day. He/she might say to a friend "Wow, on Saturday, over 50,000 people visited my site. It was my biggest traffic day [ever]". The friend could say "Why was it your biggest traffic day? Had you revamped the site? Did you have some special offers?" In this context, "it" would refer to Saturday.
The same goes for traffic on the roads. London City Council may count the number of cars on the road each day. One day could have more cars than any other day this year. 'A council spokesman said that Friday was the biggest (should be heaviest) traffic day. A reported asked "Why was it your biggest traffic day?" The spokesman replied "It was the first day of the Notting Hill Carnival, that's why." ' In this context, "it" would refer to Friday.
Does that help? Or was your question simply "Why is the word "it" missing from the sentence?"!!
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