Wow, this thread really gave me pause to stop and consider... Sorry in advance for a long reply!
In 2013, I finished a detailed Excel spreadsheet of my wardrobe inventory. I don't schedule purchases. Instead, I try to locate wardrobe holes and consider upcoming events, such as weddings, work events, or travel - situations where I might need to search out something very specific that I don't already have in my closet. The spreadsheet helps me see what I already have, so that I can try and work new items in with my existing wardrobe.
My tastes are very stable - there are certain colors, and certain silhouettes, I have loved my entire adult life. So, like Denise, I am a late-season shopper/out-of-season shopper, and this works very well for me, because if I find something I truly love, I know I will still feel that way six months later.
I think it helps that I live in the South, where there are really only two seasons: Warm-to-Hot (late March to late October), and Cold (anything below 60F). Most of my wardrobe has overlap between the two seasons.
I really only ever have two 'needs' on my list: footwear, and shorts.
For footwear, my needs in those categories are very specific: I know I am always going to wear black over-the-knee equestrian-style flat boots in winter, and ballerina flats and low-heel wedge sandals for warmer weather. I will always wear classic black and nude leather or patent stiletto pumps year-round. So when I see a current pair of shoes starting to show real wear, I take them to the cobbler, and I start stalking replacements on eBay.
To give an example of how I shop late or out of season, I had a pair of over-the-knee black boots I wore for three winters, but last year, they were pretty worn out. They went back to the cobbler, but in February, I started stalking new black OTK boots on eBay. I found my replacements in April - they originally retailed for somewhere around $435 for Fall/Winter 2013. I paid around $65, including shipping, new in their original box. I kept them in my closet until temperatures dropped in October. Since then, they have been workhorses. Shopping late allowed me to pay about 1/7 of the original retail - and it gave me something to look forward to for six months
For shorts, I have to try those on in-person, and they have been routinely more difficult to find. Shorts, for me, have to be neutral-colors (black, navy or white), and cover a range of casual-to-somewhat dressy, so when I try on a pair and love them and I know they will fit my needs, I buy them immediately.
For the rest - outwear, dresses, skirts, pants, tops, sweaters - I tend to buy items as I find and fall in love with them. Sometimes that has been cost-efficient and practical, sometimes not! I try to sit on purchases for at least two days before I pull the trigger. "Try" being the operative word.
The spreadsheet keeps me pretty honest, because I can refer to it and see whether a new item is a replacement, an enhancement, or a duplicate.
