Daytime reception issues

Earlier this year, for a about 6 months, reception on several channels was poor to non existent during weekend afternoons (either extreme tiling or no signal- "temporarily off air").  This hasn't been happening recently but has started to happen again today and is happening now. It's most pronounced on 1068 - HGTV.  The signal always returns to normal in the evening.  How is this possible that the problem only occurs in the afternoon? or only on weekend afternoons? It's happening on all boxes.