Step 1: Judge each statement on its own first.
We check the Assertion and the Reason as separate facts before linking them.
Step 2: Test the Assertion.
The Constitution of India did come into force on $26$ January $1950$, so the Assertion is true.
Step 3: Test the Reason.
$26$ January is indeed observed every year as Republic Day, so the Reason is also true.
Step 4: Look for the cause and effect.
Republic Day exists because the Constitution started on that date; the celebration is the effect, not the cause.
Step 5: Decide on the explanation.
Since the Reason is a result rather than the cause, it does not explain the Assertion.
Step 6: Select the option.
Both true, but R is not the correct explanation of As.
\[ \boxed{\text{Both [As] and [R] are true, but [R] is not the correct explanation of [As].}} \]