Thais raid is not unannounced, only the femor hills one is.
They both are random.
In thais you got these messeges, that inform you about the raid:
(Tibia wikia references)
Golden Question of AprilMarch 28, 2019
Consider adding better e-mail templatesMarch 23, 2019
Fansite item design contestDecember 31, 2018
Interview with Bolfrim - the lead tester at CipSoftFebruary 26, 2019