[Xperia Z General] katamari201:
The smartphone cycle has gotten pretty simple and formulaic if you follow it closely.

Jan-April - Interim and half-baked products and copycats. Phonemakers who can't compete on mindshare and marketing take advantage of the lull to try and stand out by releasing the newest chips and not have to compete with bigger competitors. Snapdragon 600, Tegra 3. S4 Pro. 1080p. 720p. Tegra 2. They may look enticing if you just focus on that one spec, but the overall package is usually lacking, being more rushed, uneven, poorly optimized, buggy, and will get superseded by newer, more refined versions of their chips later in the year and better components and screens overall. Being first has its downsides. Also, just a general release period for smaller phonemakers to release phones with outdated hardware to take advantage of the lull and quietness.

E.g. Motorola Atrix, Optimus G2x, Optimus 4x, Optimus G Pro, Xperia s, Xperia Z, Oppo Find 5, Lumia 920, Lumia 900

May-Aug - The big boys other than Apple start releasing their flagship products. Usually Samsung and HTC. It takes a couple months for availability to reach a global scale.

E.g. Galaxy S 1-4. HTC Sensation. HTC One X. HTC One.

Sep-Nov - Everybody starts pouring out their flagship phones. Apple announces and subsequently releases their newest iPhone. Refined tech like the Sanpdragon 800 (what the 600 should be), chips with LTE radios, higher resolution screens that don't suck, and the quick disfavor of earlier Tegra chips, all come to a head. LG screws you over by implying their earlier phones were prototypes or copycats and pulls out their real flagship phone of the year. Sony releases yet another iteration in their six-month flagship phone cycle that is marginally better than what was available earlier. HTC and Samsung release updated versions of their earlier flagships with more refined hardware. The Galaxy Note gets released, and HTC's barely phablet. Motorola releases yet another boring kevlar backed Razr using pedestrian specs with no innovation, splitting it into three products with the M, regular, and Maxxx. Nokia, like LG, also makes fun of you for buying their half-baked interim product earlier in the year and finally releases their true flagship, aligning with the late-year release of Windows Phone that FINALLY supports somewhat modern hardware. The latest Nexus device comes along. It's the holidays when retailers get out of the black and companies get their largest revenue and profit. If you are gonna release something big, can pay for the marketing, and want it to sell quickly and not rot on the (figurative) store shelves while its tech becomes outdated, this is the time. Fewer people buy stuff earlier in the year, so products with short life cycles and high R&D costs should stick to this area, if you are not a perennial favorite like the iPhone or Galaxy S.

E.g. EVERYTHING ELSE.

Roundabout answer to question. If you want to stick to a yearly phone cycle, stick with the typical Fall release that's a no-brainer and in line with everything else, from video games to TV shows to Elmo toys. The phone you buy should be good for a year at least, and you have the widest availability of phones to choose, the most refined version of the good stuff, and the manufacturing process of said good stuff has been ironed out and revved up so you won't be geting buggy 1.0 hardware components and they are churning out at maximum capacity with the widest release.