I suspect this is heard every night in the US! But here in OZ, it heralds the onset of the winter DX season. This is 3:20pm local time at Mount Evelyn:
5040 CUBA. R. Habana - Bauta. Sitting on the freq waiting for English to CAm, noted the f/in at 0520, first time for this winter DX season. Rapidly improving strength by 0525. Scheduled s/off is 0600. Apr 29.