Go through your results for flights to Lille in France with Netflights. Although Air France usually has the best prices, it's best to double check what's available for your travel dates.
Flexible on dates? Conducting your search by month allows you to compare prices to Lille throughout the year. Accommodation prices will also be higher at this time of year.
Look out for flights with Air France, as they often have good deals. Or broaden your search by looking at other airports in France.
Looking at a few different departure airports will give you even more choice. See what you can find. And remember, it's best to book your flights in advance wherever possible.