Part of the challenge of planning a party dealing with the 'what ifs.'
And probably the number one concern that most people have when planning an event is bad weather. Bad weather can gum up the works in a number of ways.
If the event is in the winter, a harsh storm can make driving treacherous or impossible. I recall a blizzard several years ago that was severe enough for governmental authorities to shut the highways to all but emergency vehicles.
If your event is in the winter months, it behooves you to raise the issue of inclement weather to your vendors. Each vendor will have its own policy on policy on cancellation. In some cases you may lose your deposit. Some vendors such as caterers have to incur up-front out-of-pocket costs to serve you and they cannot be expected to absorb them.
Some vendors will apply your deposit to your new date.
If your event is an expensive one such as a wedding, consider wedding insurance. It is designed for such occurrences (but not for couples who get cold feet).
In some scenarios, a rain or snow date will work well.
The downside is that outside vendors and venues have to keep themselves open for two dates instead of one. Some may not be able to accommodate that or there may be additional charges. Also few guests are likely to commit to two dates upfront.
In warmer months, people living in hurricane prone areas have to deal with that threat. The same scenario applies as a blizzard in more northerly climes.
But rain can disrupt outdoor events any time of year. Roads can be flooded out in heavy rain and that can delay your vendors and your guests.
It is delightful to be outdoors during the spring and summer. But ill-timed rain can gum up the works if you have not planned for its possibility.
The easiest and least expensive way to handle that contingency is to have an easily accessible, nearby sheltered area. That could be a nearby building, a pavilion or a tent. Then the 'show' just goes on rain or shine.
Most hotels and catering facilities are set up for both indoor and outdoor scenarios. Many public parks have pavilions that you can reserve or rent at little or no cost.
And if the event is at home, just keep some rooms cleared in case the party has to make a quick dash indoors.