Colds are more common in colder months for several reasons. People spend more time indoors in closer proximity, which makes it easier for respiratory viruses to pass from person to person. Indoor heating dries the air, and dry conditions can help some viruses linger and spread more readily.
Drier air may also dry out the protective lining of the nose, which can make it slightly easier for viruses to take hold. Reduced daylight and other seasonal factors can play a role too.
Good habits reduce your risk year-round: frequent handwashing, avoiding touching your face, ventilating indoor spaces, and staying away from people who are unwell where possible. Staying up to date with recommended vaccinations such as the flu shot helps protect against influenza specifically.