The advantage is simplicity and auditability: a date arrives, you trade, and there is no judgement involved. It also avoids the trap of checking daily and finding a reason to act on every wiggle.
The disadvantage is that the calendar has no relationship to market movement. An annual schedule might leave a portfolio 12 points overweight equities for eleven months, then rebalance the week after a crash. Hybrid rules are common: check quarterly, but only trade if a band has been breached.
Example: a portfolio checked each 31 December. In a year where equities gained 30% then gave it all back by November, the calendar rule captures nothing, while a band rule would have trimmed into the rally. See rebalancing-bands.
Related: rebalancing, rebalancing-bands, portfolio-drift, asset-allocation, tax-aware-rebalancing