General:
* Prompt
* Punctual
* Opportune
* Seasonable
* Well-timed
* Appropriate
* Expedient
* Relevant
More specific:
* Urgent: For situations where immediate action is required.
* Fortuitous: For something happening by chance at a lucky moment.
* Propitious: For something happening at a favorable time.
* Strategic: For something happening at a planned moment to maximize impact.
* Critical: For something happening at a crucial point.
More informal:
* Right on time
* Just in the nick of time
* On the dot
The best word to use will depend on the context of your sentence.