Avoid complications and unknown factors. Just about every bride has a trail make-up application well before the wedding. This is important as the make-up has to suit the wedding dress, and often there are bridesmaids and location photography to facto in as well. If you are considering eyelash extensions the best time to try them is a few days before the make-up trial. The make-up will be at least a little different if you have longer eyelashes, and in the unlikely event of an allergic reaction at least you will know what to avoid on the big day itself.
If the lashes are put on a few days prior to the trail make-up it will give the individual some time to get use to them. After avoiding water and steam for a day or two relatively little upkeep is required. It is only really the sudden change from short to long lashes that causes a little awkwardness. If your lashes naturally grew to this length over a few weeks we would accommodate it more gradually.
Do tell the make-up artist in advance that you have eyelash extension. It is better to use non-waterproof make-up around the lash and eyelid as it allows the skin to breath and the natural lashes to grow.
Get the lashes topped up (i.e.: get inlays) a few days before the wedding itself. This will prevent any noticeable gaps caused by the individual lashes naturally falling out over time.
Always make sure the inlays use exactly the same glue and materials that were used the first time, lest a different material causes an unexpected reaction.
Above all, always have the eyelash extensions done by a certified professional, and return to the same professional for the regular inlays.