If "observe" means to watch or notice:
* Ignore: To pay no attention to something.
* Overlook: To fail to notice something.
* Disregard: To treat something as unimportant.
If "observe" means to follow a rule or custom:
* Violate: To break a rule or custom.
* Disregard: To ignore or disrespect a rule or custom.
* Defy: To openly resist or disobey a rule or custom.
If "observe" means to make a remark or comment:
* Remain silent: To say nothing.
* Keep quiet: To not speak.
* Suppress: To hold back from expressing an opinion.
If "observe" means to celebrate an event:
* Ignore: To not acknowledge or celebrate an event.
* Neglect: To fail to pay attention to or celebrate an event.
* Disregard: To treat an event as unimportant.
To choose the best antonym, consider the specific context in which "observe" is being used.