To notice or view, especially carefully or with attention to detail.
To take note of and celebrate (a holiday or similar occurrence); to follow (a type of time or calendar reckoning).
To comment on something; to make an observation.
To follow or obey the custom, practice, or rules (especially of a religion).
To examine something with great care or detail, as to look for hidden or obscure flaws.
To audit accounts etc in order to verify them.