Work out the date of the fourth Saturday in each month, if that date has passed, tell me when the next one is. 

if (date('j') <= date('j',strtotime('Fourth Saturday '.date('F o')))) {
	return "The next market is on the ".date('dS F',strtotime('Fourth Saturday '.date('F',strtotime('Next month')).' '.date('o'))); 
	} else { 
	return "The next market is on the ".date('dS F',strtotime('Fourth Saturday '.date('F o'))); 
	}
}