Fun fact: I have a TextExpander snippet to write the Throwback Thursday text, that automatically includes the appropriate week number and year, thusly:

#! /bin/tcsh
set w=`date +"%U"` ; set y=`expr 1969 + $w + 1` ; echo "For week/year $w of my 52 years celebration, from ${y}: . #ThrowbackThursday #52years"

Which outputs:

For week/year 31 of my 52 years celebration, from 2001: . #ThrowbackThursday #52years