The problem is almost never the tan. It is the tan line at the neck and the blouse.
A tan is your skin protecting itself. It fades as the skin renews, which takes roughly four weeks, sometimes longer on the arms and neck where skin is thicker.
In photographs, an even tan is almost invisible. An uneven line is not. So the whole job is avoiding the line, not chasing the colour.
Outdoor haldi, mehndi and pool functions: sit in shade. Shade beats every cream on this page. Wear sunscreen on the neck, ears and the back of the hands. These are the three places everyone forgets and the camera never does. If you are doing an outdoor function, wear something close to the neckline of your wedding blouse. That way any line lands where the blouse will cover it.
Time and sunscreen. Four weeks does most of it. A gentle de-tan clean-up at a salon you trust, no closer than ten days to the wedding. A good moisturiser. Dry, tanned skin looks darker than moisturised, tanned skin.
Bleach on the arms a week before. It does not remove tan. It irritates skin, and irritated skin darkens further. Hard scrubbing. Scrubbing removes the top layer unevenly. That is how a patchy look starts. Multiple de-tan sittings in the last week. Skin does not renew faster because you paid more.
Shade at every outdoor function. Sunscreen on neck, ears and hands. Wear a similar neckline to your blouse during outdoor functions.
About four weeks for most skin, longer on the arms and neck. Daily sunscreen is what lets it fade instead of topping it up.
A gentle de-tan clean-up can even out the top layer a little. It does not undo a tan, and repeated harsh sittings close to the wedding do more harm than good.