The Activity Stream shows the tag Yesterday for activities performed between 24 and 48 hours before, but it is not totally right.
The tag Yesterday should be applied only to those activities performed between the midnight and the midnight of the day before.

You can see an example in the attachment: today is Friday and the Activity stream is showing different time tags (‘Yesterday’ and ‘Wednesday’) for activities performed during the same day (Wednesday) because it is counting yesterday starting from the 48 hours before. When you go to the ‘View issue’ screen of an issue tagged as ‘yesterday’ it shows the right date (Wednesday, which is not yesterday).