It is best avoided, and for new work you should not do it. Copper and aluminium behave differently where they meet: they expand and contract at different rates with heat, and aluminium quickly forms a hard, insulating oxide layer on its surface. A plain copper-to-aluminium joint therefore tends to loosen and develop resistance over time, which makes it heat up — a well-documented cause of connection failures and electrical fires.
If the two metals genuinely must be joined (for example, connecting to an existing aluminium service), it has to be done properly: with bi-metallic connectors or Al–Cu rated lugs designed for the purpose, treated with an anti-oxidant compound, and torqued correctly by a qualified electrician. Ordinary twist joints or a single lug crushing both metals are not acceptable.
For a fresh installation, keep the whole circuit in a single metal. Copper is the usual choice for home wiring because of its higher conductivity, better flexibility and easier, more reliable terminations; aluminium is generally reserved for larger, cost-sensitive fixed runs sized up to compensate for its lower conductivity.