Grate the cucumber on top of a clean kitchen towel or cheesecloth. Ring out the excess water over the sink, then transfer it inside a medium bowl.
Grate the garlic into the cucumber bowl, add the yogurt, lemon juice, dill, and salt. Mix to combine.
Refrigerate, covered, up to 2 days or until ready to serve.
Make the pita sandwiches:
Lightly warm up the pitas in the microwave for 30 seconds, in the oven at 300F for 6 minutes, or in a toaster oven. You want to lightly warm or toast them so they fold easily and taste better.
Once warm, spread about a tablespoon of tzatziki on top of the pita in the middle. Place a layer of salad greens over the sauce, add sliced tomatoes, and a few cucumber slices.
Arrange the falafels on top of the veggies, either cold or warmed, your preference. Drizzle additional tzatziki sauce over the falafels and fold the pitas, tucking one side under the other. Use a toothpick or wrap in wax or parchment paper to hold the pita sandwich closed.
Notes
You'll find ready-made falafels at many grocery stores, often in the frozen section. You can also make this air-fryer falafel recipe or this paleo falafel recipe with lots of veggies.