Ultimately, when you invoice depends on your specific industry and business model. For smaller projects or mini sessions, you may opt to send a single invoice for full payment prior to the event, especially if the session is part of a promotional offer or sale. If you’re invoicing for a larger event or for a session that involves unknown travel expenses, you may want to send an initial “retainer” invoice followed by the full invoice after the gig.
Some photographers recommend collecting half of the payment upfront and the other half upon delivery, whereas others will only ever work with clients who are willing to pay in full before the session. Do what works best for your business, and collect as much payment up front as you are comfortable asking for.