Mailing mail before the deadline does not guarantee that it will be postmarked on time due to operational changes by the USPS. These changes include the consolidation of processing centers and adjustments to collection and transportation schedules. This means that mail often travels further before being sorted, which could delay the actual postmark date.
Previously, mail was more likely to be processed locally and quickly, often on the same day. Now, mail may sit longer between pickup and processing, or get routed through regional facilities, delaying the official postmark. Plus, collection times in many areas have shifted to earlier in the day.
All of this means that even if you drop off a letter on, say, April 15, it might not be processed, and therefore postmarked, until April 16 or later.