Aalsmeer and Keukenhof

It is possible. Aalsmeer is not that far from Keukenhof. Most of the action on Aalsmeer flower auction occurs very early in the morning between 7 and 9 AM. From there you can go to Keukenhof and enjoy the rest of the day since it is opened until 7 PM. Easiest way is to rent a car for the day (that would allow you to do some leisure driving through the tulip fields and the dunes nearby as well) Using public transportation, you need to catch a train to Schiphol, then 2 buses to Aalsmeer (whole thing takes 50 minutes). Then 2 buses back from Aaslmeer to Schiphol and shuttle bus from Schiphol to Keukenhof. Then shuttle bus back to Schiphol and train back to Amsterdam.

When Googling, it's helpful to know that the flower auction company is FloraHolland. Their website says that Connexxion bus 172 goes directly from Amsterdam to Aalsmeer. It appears to be a very frequent service: http://www.connexxion.nl/dienstregeling/lijn?id=M172&richting=2 There are four stops at FloraHolland, a very big place. I'm sure the driver can tell you which one is the Visitor Center - a dedicated entrance. We had a car, which was convenient. But traffic around Keukenhof is very, very heavy during the short Keukenhof season. If anything, busses might get some slight entry preference, so don't lament your need for public transportation. The grass parking lots are no pleasure on a damp day. I'd consider a tour company excursion from Amsterdam. However, looking at the Keukenhof site, you could consider going Aalsmeer to Schiphol on Connexion 858, and return to Amsterdam with the two busses. Do not skip Aalsmeer. It is fascinating. Their website mentions a local garden and a local boat ride, but we didn't do them.

"The grass parking lots are no pleasure on a damp day". I found that out the hard way...

@Tim: using a train to Schiphol + 2 buses to Aaslmeer works faster than the "direct" (but much slower and subject to traffic) buses. There is no reason to go with a tour to Keukenhof. Shuttle buses from Schiphol depart often. There are 8-11 trains per hour between Amsterdam (Centraal or Zuid) and Schiphol.