I’ve been watching a particular stock, and it seems that for most days its volume increases dramaticaly in the last 30 minutes of the trading day. Next morning and day the volume is low again until the last 30 minutes, etc.
Is there a reason for this? Is this ETF rebalancing going on or something?
that’s pretty much it…re-balancing and mostly Mutual Fund orders ( which are placed at the end of the day)
While it could be rebalancing, it could also be a sign the stock is under accumulation (or distribution). I have invested in stocks for many years and follow a Technical Analysis philosophy. The reason it could be under accumulation (for example) could be that some insiders know something and are slowly accumulating stock so as not to hit the price up suddenly…be ‘under the radar’.
…or it could be a mutual fund/ETF that has decided the stock is worth getting but accumulate slowly so as not to attract attention.
I can’t be more judgmental about it unless I know the stock, something I presume you don’t wish to share.
Volume across the whole market, not just your stock, follows a U-shape throughout the day. Trading is heavy in the morning, dies down in the afternoon, and picks up again towards the close.
But not ‘dramatically’ usually.
Eh…I’d say yes, pretty significantly. I’d say it’s not too controversial to say that the first hour and last hour’s volume is, on average, about double the average volume of the midday hours.
If his version of ‘dramatically’ is about double…I will box his ears! 
Here is the volume over last 10 days in 15 min intervals.
There are several days where there is way more in the last 15 min of the day. Even the days that show less of this effect are easily 3-4 times more than mid day hours.
