With some planning, a service call fee and a little bit of luck, this can be done without harming your instrument. Whether it's a wedding ceremony, cocktail party or family reunion, pianos end up in yards sometimes.
Here are a few tips:
First, contact your piano mover to estimate the cost of moving your piano to the yard and then back after the party or the next morning.
They can usually recommend a piano tuner that can come and tune the piano on the day of your event. Most tuners charge a little more to do this type of pre-event service call.
If this all works out, plan on buying or renting a small canopy that is used for weddings and special yard events. It's not a bad idea to pick up a ground cloth, the type that you use under your tent when camping.
Now, schedule the movers to move the piano out to the yard, early on the day of the event.
Schedule the piano tuner to come at the same time and have them tune the piano after it's in place under the canopy.
Keep a soft blanket and the ground cloth folded up near the stage area just in case of a passing shower. The piano should be fine under the canopy, but you may want to cover it with the blanket to keep the ground cloth from scratching the piano.
Of course, if there is a 90% chance of rain on the day of the event, it's a good idea to scrub this idea...period. (If rain hits your piano directly, it can ruin it forever.) Check with the movers and the tuner to see what their policies are on cancelling a service call.
Many times, in many towns all over the world, people move their pianos outside to hold special events...even when there is a slight chance of showers.
After the event is over, cover the piano with the blanket and the ground cloth and wrap it once or twice with light twine or a small rope. Also, you may want to pick up a spray bottle of rodent repellant and spray it on the ground a few feet from the piano area before you call it a night. (If mice or chipmunks get inside your instrument, it can cost hundreds of dollars to fix.) The piano should be fine under the canopy for the night.
The movers should be scheduled to come back first thing in the morning to return the piano to your living room. Chances are that the piano may need another tuning after it is moved back in the house. This can be done the same day or weeks later, it's up to you.
If you plan carefully, a memorable time can be had by all with live cocktail piano and sing-alongs right in your own back yard without damaging your piano.
It can be done, but be careful.