Phineas decides to beat the heat by conducting a TV Christmas special on a hot summer day. In it, Isabella sings "Let It Snow! Let It Snow! Let It Snow!" which was written by Sammy Cahn and Jule Styne in 1945 during one of the hottest days on record.
This is the only 11 minute episode to be produced without a partner. When it's shown in reruns, it's usually either paired up with "S'winter" or shown at the end of the hour when "Phineas And Ferb Christmas Vacation" is showing.
The calendar on the wall shows Christmas occurring on a Wednesday. This would put the year not at 2011 (the year of this episode's release) but either at 2002 (or some year earlier) or at 2013 (or some year later).
Though this Christmas episode supposedly takes place in mid-summer as a "live" broadcast, it didn't air till December 2, 2011, which would make it time-appropriate.