Maui does not really have a bad season. It has trade-offs.
April through June is the sweet spot for most couples. The weather is dry and warm (80 to 85 degrees), trade winds keep things comfortable, humidity is low, and it falls between the winter tourist rush and summer family vacation crowds. Venues have better availability and some vendors offer off-peak rates. Sunsets are around 7pm, which gives you a nice evening ceremony timeline.
September through November is the other window we recommend. Similar weather, even fewer tourists, and hotel rates drop. The catch is that September and October are the warmest months, and there is a slightly higher chance of brief afternoon showers. Nothing that ruins a wedding, but something to plan around if you are doing an outdoor ceremony.
December through March is whale season, which sounds great and is great if you want humpback whales breaching in the background of your photos. It happens. We have seen it. But this is also peak tourist season, so venues book 12 to 18 months ahead, hotel rates are highest, and flights are more expensive. Rain is more frequent, especially on the north and east shores. South Maui (Wailea, Makena) stays drier year-round.
July and August work fine but it is hot, busy, and more expensive. If your guest list includes families with school-age kids, summer might be the only option, and that is okay. Just book early.
A few things people forget: Maui has microclimates. Wailea can be sunny while Hana gets four inches of rain. Pick your venue location based on weather patterns, not just scenery. Also, sunset time changes by about 90 minutes across the year. In December it sets around 5:45pm. In June, around 7:15pm. That affects your ceremony timeline more than you might expect.
We have planned weddings in every month of the year. Every single one turned out fine. The best time is the time that works for you and the people you want there.