Use the dates your photos were taken on to find them using a calendar.
CalendarView is a module for Gallery 3 which will allow users to navigate your photo albums using a calendar. Calendars are automatically generated with links to the photos, based on the EXIF date the pictures were taken on.
This module may be downloaded from the Gallery forums here.
To install, extract the "calendarview" folder from the zip file into your Gallery 3 modules folder. Afterwards log into your Gallery web site as an administrator and activate the module in the Admin -> Modules menu.