When to Come....
The usual time to come to Crete is between late April and late October.  The main reason for this is often cited as the weather, but this is usually good at any time of the year.  Crete averages around 300 days of sunshine every year, though there are no guarantees with the weather!  The real reason for the seasonal arrival is probably that most people in northern climes take their holidays during spring and summer, and the holiday companies, following the trend, lay on direct charter flights during these months.  Making getting here from most parts of the UK something like 3.5 hours.  In reality the travel often amounts to much more because of the travel involved in getting to the airports and the two hour check-in time before an 'International' flight.  But when to come is really a bigger question.  It depends very much on what you want.

One of our friends, a keen bird-watcher, arrived here a couple of years ago complete with his books, photographic gear and enthusiasm, but despite his daily early starts and late stops saw almost nothing at all in the bird line!  I suspect that his wife had picked the time - she just loved the sunshine!  By contrast I am no stranger to many species, Golden eagles, various hawks, very colourful small birds I don't know the names of; buzzards; vultures and the owls, both scopes and barn.  I spend some of my evenings watching owls on nearby roof-tops for several months of the year without really noticing the dates.  Last year we were even able to watch the offspring of the owls emerge and make their first flights. 
Our night sky is sometimes plain; sometimes absolutely filled with stars.  The local wild-flowers arrive and depart in profusion, but not in late summer and especially not August when the countryside turns parched brown and is often in danger of catching fire and becoming parched black.  Some countryside is dangerous at certain times of the year.
Sea, Sun and Sand...
If you are looking for a holiday-making sea-side visit then nip into your local travel agent either in person or via the internet, have a good look at the expected temperatures at any particular time of the season.  Ask yourself whether it will be too hot for babies, young children or elderly relatives and take your pick.  Go middle east for typical sea-side resorts and far East or North West for increasing solitude and more basic facilities.  If you choose accommodation with a pool temperature doesn't matter as you will probably have air-conditioned rooms next door.  Strangely, such accommodation is often next to the sea.  Best of both worlds.  Always, but always, ask if there are busy roads to negotiate, especially if you have small children.  Push-chairs and main roads do not mix.
Specialist Holidays...
Many travel companies will take care of when is the best time to come by offering specialist holidays.  Having said that remember again that these may be general.  Wild flowers may well bloom for a few months, but do the varieties that you are interested in?  The night sky does change.  Cycling holidays can be taken at any time.  But what about the other factors such as the ubiquitous sun.  Share your knowledge with your travel agent and you might just get a match, but don't expect an agent to know everything.

The alternative is to come whenever YOU want to.  Even outside the holiday season there is no problem.  Scheduled airlines will get you to Crete at any time of the year.  From the UK Olympic; and Cronus/Agean will book you all the way here, sometimes at very good prices.  Sometimes easyJet, who only fly as far as Athens can provide unbelievably low prices - even when you add a local return to Athens via Olympic or maybe Agean.

To make sure that you come at the right time ask yourself what you want to see and do; whether the weather at the time affect any companions adversely; and how you want to get here.  The page on getting here offers more information about this.