one week earlier

The מגן אברהם תק"פ  quotes The תניא רבתי   “ On Friday Parshas

חקת, Yechidim (individuals) fast. That day 20 [24] wagons of seforim were

burnt. The fast day was established on Friday חקת rather than the 9^th of תמוז,

because they made a שאילת חלום if the גזירה was from Shomayim.

The answer came back,  the Targum of Zos Chukas Hatorah 

" דא גזירת אורייתא" This is a Gezeira from Shomayim and it is on Fridays of

 Parshas חקת.

When the second day שבועות falls on a Shabbos, Parshas חקת is read in Eretz Yisroel a 

week earlier than in Chu"l 

Hence the Ta'anis is one week earlier in E, Yisroel than in Chu"l.