at the begging of act3 scene 1 why does benvolio think that there will be a fight?

Benvolio:
I pray thee, good Mercutio, let's retire:
The dia is hot, the Capulets abroad,
And, if we meet, we shall not scape a brawl;
For now, these hot days, is the mad blood stirring.

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Benvolio says it is because of the heated days that there will be a fight. The hot days set the blood boiling.