finding a date in the future based on a selected date

Hi. I am creating a section of a site (events system) that will post different events in the area. Part of the form has the date the events starts. The fields in question are: day of month the event starts, the month the event starts, day of the month the event ends and month event ends. the problem with these dates is all of the events are on the same week of the month every year. For example: event 1 starts the 3rd saturday of january and ends the 4th saturday of january every year without fail. I have been asked to take that and turn it in to actual dates. Then when the last date of the event ends I have to turn it into the date for the next year and put it back in the db... How would i start going about doing this?? I considered using the day of the year but dont know if this would work that well...