* You buy Turnips from "Kaburiba", every Sunday morning, up to 12 AM. * Kaburiba asks you how many turnips you want to buy (you just input a number). * Turnips can stack up to 990, they are sold in units of 10.
* You must sell them at R-pacas (not Nook) before they spoil by next sunday * Each day has two prices, a morning price and an afternoon price.
As for the "spikes", the prices follow a number of different patterns, each pattern starts and ends within one week (mon-sun), and as far as I can tell, each week is independent from another.
Some possible patterns: - Lowering pattern: selling prices start around the buying price, and then go steadly down by small amounts. - Peak pattern: same as lowering pattern, but with a huge spike (400 or more) Thursday afternoon - Random pattern: prices are completely random, between 50 and 200 bells - "small peaks": similar to lowering pattern, but somewhere in the week there will be a "small peak", with prices up to 200-300 bells.
I think there may be other patterns, the Japanese wiki has a very complete description of them.
Check out Liquefy's Animal Crossing: City Folk guide, and just go straight to this section: "26) The Stalk Market". He's listed a number of useful stats and typical price quote patterns.
Don't forget, you really need to be able to check the prices twice a day. Also, the morning price is only good between 9 am and noon, which can be awkward if you have to sell all your turnips right then.
I've been playing since launch day, and so far, I've only done poorly with turnips in one week. 65% of the time I've correctly identified the highest price, and rest of the time I was very close.