Unlock schedules are published in advance, so the date is not a surprise. The market's handling of it often is: price frequently weakens into a large unlock and can rally afterwards once the overhang is known.
Not every unlocked token is sold. Early investors with a low cost basis are the most likely sellers; team tokens may be held or hedged. But unlocked supply can be sold, and that optionality alone affects price.
Example: a 6% of circulating supply unlock at a $400m cap is $24m of potential sell pressure against maybe $8m of daily volume, three days of volume arriving at once. Check unlock calendars before holding through one.
Related: vesting-cliff, circulating-supply, fully-diluted-valuation, tokenomics