A casual survey will usually find that many people believe the phrase is in reference to the conspicuous laziness of domesticated dogs (who are in danger of overheating with too much exercise) during the hottest days of the summer. When speaking of "Dog Days" there seems to be a connotation of lying or "dogging" around, or being "dog tired" on these hot and humid days. Although these meanings have nothing to do with the original source of the phrase, they may have been attached to the phrase in recent years due to common usage or misunderstanding of the origin of the phrase.
— thx, wikipedia