The Tribulation starts straight after the Rapture with the first 6 Seals (Revelation 6). After an initial period in which the 144,000 are prepared (Revelation 7), the 7th Seal is broken releasing the 7 Trumpets (Revelation 8:1-2). This coincides with the start of Daniel's 70th Week, and the start of the Ministry of the 2 Witnesses.