Where

The building is on Central Avenue.
Lena works in the building. --->

The building where Lena works
is on Central Avenue.*

Note:

The sentence with where has an adjective
clause: where Lena works. In the clause,
where is used in the same way as a relative
pronoun. The clause modifies the building.

_________________________________

The city is about 200 miles north of here.
Joe's family lives in the city. --->

The city where Joe's family lives is about
200 miles north of here.*

Note:

The sentence with where has an adjective
clause: where Joe's family lives. In the
clause, where is used in the same way as
a relative pronoun. The clauses modifies
the city.

_________________________________

They live in a building. There are many
apartments in the building. --->

They live in a building where there are
many apartments.*

Note:

In the sentence above, the adjective clause
is where there are many apartments.
The clause modifies a building.

When

I'll always remember the day.
We met on the day. --->

I'll always remember the day
when we met.*

Note:

In the sentence above, the adjective
clause is when we met. The clause
modifies the day.

_________________________________

Autumn is the season. The leaves
of many trees change color during
the season. --->

Autumn is the season when the leaves
of many trees change color.*

Note:

In the sentence above, the adjective
clause is when the leaves of many trees
change color. The clause modifies
the season.

_________________________________

Thanksgiving is a holiday. Families like
to gather together on the holiday. --->

Thanksgiving is a holiday when families
like to gather together.*

Note:

This time the adjective clause is when
families like to gather together. The
clause modifies a holiday.

Do not use prepositions before either
where or when in an adjective clause:

wrong:
*The building in where Lena works
is on Central Avenue.

wrong:
*They live in a building in where
there are many apartments.

wrong:
*I'll always remember the day
on when we met.

wrong:
*Autumn is the season during when
the leaves of many trees change color.

In very formal writing, you may, however,
use prepositions before which--but not
before that:

good:
The building in which Lena works
is on Central Avenue.

wrong:
*The building in that Lena works
is on Central Avenue

____________________________

good:
They live in a building in which
there are many apartments.

wrong:
*They live in a building in that
there are many apartments.

____________________________

good:
I'll always remember the day
on which we met.

wrong:
*I'll always remember the day
on that we met.

____________________________

good:
Autumn is the season during which
the leaves of many trees change color.

wrong:
*Autumn is the season during that
the leaves of many trees change color.