Weeds generally can be killed any time of the year, but the best time is when they are growing strongly as they will absorb weed killer quicker.  One of the quickest ways to take care of weeds is the use of chemicals as they take the backache and hard work out of weeding, and are much more efficient getting rid of weeds.  Systemic weed killers with glyphosate will kill weeds to their root, even those stubborn deep rooted weeds.  Make sure you follow all instructions on the container for best results and safety