Do you all believe in the December sell-down effect? Some investors believe it is good to buy shares in December, thoughts?

Investors always love to find trends. Can you believe there's a almanac for stock traders book? But all in all, it's based on past trends with evidence to support - i.e. wanting to cash out to buy stuffs during boxing day / Christmas presents. Fund managers going on holiday, hence selling the funds that require monitoring. You could take it with a pinch of salt nonetheless, like how everyone has been saying for a year now that a recession is going to occur but once it occurs, they will go like "I told you so! / I forecasted this would happen!"

Best is to not wasting time on stuff that is out of your control.

December is the tax reporting and holiday period. Most traders I know from the US will take a break in December too. Volume will be lower during this period.

No nobody knows when the market is up and down. It is due to Mr market's temperament of the day.

If one could forecast, one would already be a billionaire and not be typing on this forum