Find the Perfect Timing
There are lots of ways to reduce an event's environmental footprint, and one way is to pick a time to hold the event that makes it easier to use fewer resources. For example, choose a month when the weather is mild, so you won't need to heat or cool your venue. You can also hold your event in the daytime so you won't need as much electricity to light everything up.
Maybe your perfect timing involves a point in the week or day when there's not a lot of traffic so people won't spend as much time on the road trying to get there. Perhaps the starting and ending times of the event could coincide with bus or train schedules so it's easy for people to use mass transit to arrive and leave. You might pick a time of year when local foods are being harvested, so the food you serve won't need to travel as far to get there. You could even avoid hot times of the year or day so people won't be tempted to drink lots of bottled water.
Just remember that while it's good to consider the environment when choosing a date and time for your event, you shouldn't completely neglect the needs of your attendees. Your perfect timing will be a balance of energy- and resource-saving measures, and convenience for your guests.