Sun Protection Factor, or SPF, protects your skin against harmful UVA and UVB rays. Since sun damage can occur even in short periods of sun exposure, it's a great idea to apply sunscreen every morning—it will become second nature before you know it.
Some skincare ingredients make your skin more sensitive to the sun, making SPF protection even more important. For example, if you happen to be using a natural retinol serum, using a daily SPF is imperative. Retinol enhances cell turnover, which makes your skin more prone to UV radiation. The retinoid effect lasts for days after application, so it's best to protect your skin daily and not just the day you've used retinol.
Additionally, if you're planning on spending some quality time outdoors, work in moisturizer and sun-specific makeup with SPF for added protection.