Today VIX is computed based on near- and next- term options series which fall into the time period of [23, 37] days. That is what it is now, when they use SPX weekly, so they have options expiring every week.
The question is, how was VIX calculated before 2014, when they used traditional SPX options only, expiring every month on the 3rd Friday? Could there be a situation when VIX was computed based on options with maturities of 1 and 31 days, for example?
And why end-of-month series are ignored?
Answer
There were two changes to the VIX; the first change in 2003 that switched from S&P 100 options to S&P 500, and from implied volatility to variance swap method. The second change was in 2014 when calculation included weekly options.
Before 2014 the first series used had to have at least one week to expiration. Then the next series was used without any limit on time to expiration.
Month-end options were certainly considered, but are not used probably because historically they have been less liquid than regular 3rd Friday options. It is hard to say why CBOE did not include them when they included weeklys - I have not calculated statistics, but I just checked quotes, and bid-ask spreads, volumes, and open interest on month-end options appear comparable to nearest weekly options.
Source: pre-2014 VIX white paper http://www.dormantrading.com/uploaded/docs/directory/vixwhite.pdf
Alternative (reliable) link to white paper: https://web.archive.org/web/20091231021416/https://www.cboe.com/micro/vix/vixwhite.pdf
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