Some businesses have products that need to be promoted during certain times of the year in order to raise awareness and boost sales during their busy period. For example, if your business sells products or services that are used during summer then you will need to start marketing these in spring as the weather starts to warm up.
If you have products that are going to feature on a TV show or in a radio chat then you may want to start working on your marketing at least a few weeks before-hand and check that your website has all the relevant information on as you may find a sudden increase in traffic.
With marketing campaigns that are time sensitive, you should work backward from your deadline to ensure that you know what needs to be done and what in order to be ready on time.
It may be that you can do a lot of the leg work beforehand such as setting up Adwords campaigns and then just make any final adjustments and set it live when you need to.
If you work with a marketing agency be sure to let them know any important dates you have coming up well in advance to give them time to make a plan and get the work done.