Facebook started showing the weather on event pages:
How to switch temperature from Fahrenheit to Celsius? (or from Celsius to Fahrenheit)
Solution for October 2014 (if you don't see a "Today" dropdown as in the marked solution)
Go in the Settings page of Facebook:
https://www.facebook.com/settings
(OUTDATED see firepol's solution)
Go to your Events page
In the upper right, click the gear:
Switch to Celsius:
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At least for me (I see facebook differently, maybe because I'm a developer?) now, October 2014 I don't see any "Today" dropdown as shown in point 2 and it's impossible to me to find a "Settings" page to manage the Events temperature... – firepol Oct 8 '14 at 12:00
