Here's what you need to know:
* There isn't one specific episode where he wears glasses and it becomes a recurring theme.
* He has worn glasses in some episodes for plot-related reasons. For example, he might need them to see something better, or he might be injured and require them.
* Matthew Gray Gubler, the actor who plays Spencer Reid, wears glasses in real life, so it's possible they were incorporated into some scenes for realism.
If you remember a specific episode where you think Spencer Reid was wearing glasses, it would be helpful to know the season and episode number to be able to confirm that.