History
The Most Serene Republic of Venice dates back to 827, when a Byzantine Duke moved its seat to what is now known as the Rialto, and for the following 970 years, prospered on trade and under the rule of a Roman-style Senate headed by the Doge. In 1797, the city was conquered by Napoleon, a blow from which it never recovered. The city was soon merged into Austria-Hungary, then ping-ponged back and forth between Austria and a nascent Italy, but Venice is still a monument to the glory days of the Renaissance, and historical culture still throbs powerfully in the old Italians' veins.
Climate
The summer may be the worst time to visit: it's sometimes very hot and often humid, the canals usually smell, there are mosquitoes and occasional infestations of flies, and there are more tourists than usual. Spring and fall are probably best, a compromise between temperature expect 5-15°C in March and the tourist load. Between November and January, you may manage to feel you have Venice all to yourself, an interesting and quiet experience. Beware of the weather during the winter months: it can be quite cold, windy, and damp. Fog is an additional hazard if you are driving in or out, doubly so in the unlikely chance that you will pilot a boat. That said, if you've never been to Venice, it's better to go in summer than not to go. You won't regret it. Many cities are far worse in summer, and Venice has no cars, hence no smog.
Acqua alta high water has become a fact of life in Venice. The lagoon water level occasionally rises above the level of the squares and streets, flooding them. This can happen several times a year, at irregular intervals, usually in the colder months. Acqua alta usually lasts a few hours and coincides with high tide. You'll see raised walkways in side alleys ready to be pulled out when acqua alta hits. When the city begins to flood, sirens will sound to warn residents and businesses. If you speak fluent Italian, tune into news programs since their predictions of the times the flood begins and ends are usually on the spot. Normally, the tide rises and falls in six-hour cycles.
You can get an acqua alta map at the tourist offices either at the railway station or St Marks. This will show you the higher, dry routes and the ones with walkways setup during the various flood alerts. There is a tide measuring station at the Rialto vaporetto piers, and a noticeboard at the base of the Campanile in the Piazza San Marco that shows a live tide reading and predictions for the next few days.