33 USC 2020 - Application (Rule 20)

(a) Compliance in all weathers 
Rules in this Part shall be complied with in all weathers.
(b) Rules concerning lights complied with from sunset to sunrise; other lights 
The Rules concerning lights shall be complied with from sunset to sunrise, and during such times no other lights shall be exhibited, except such lights as cannot be mistaken for the lights specified in these Rules or do not impair their visibility or distinctive character, or interfere with the keeping of a proper lookout.
(c) Lights during daylight hours in restricted visibility; other circumstances 
The lights prescribed by these Rules shall, if carried, also be exhibited from sunrise to sunset in restricted visibility and may be exhibited in all other circumstances when it is deemed necessary.
(d) Rules concerning shapes; compliance by day 
The Rules concerning shapes shall be complied with by day.
(e) Annex 
The lights and shapes specified in these Rules shall comply with the provisions of Annex I of these Rules.