I've had many memorable birding days in the nearly 13 years since I officially started my life list. Here are some of the bigger contenders:
November 20, 2004: the day of my first pelagic trip (from Ventura Harbor around the northern Channel Islands of California), during which I saw lifer #100 (Western Grebe) in the harbor, one of 20 lifers seen that day.
March 13, 2010: birding tour of the King Ranch near Kingsville, Texas, which I visited with other wildlife students from Humboldt State University, during which I saw lifer #300 (Ovenbird) and 23 others, plus one more (White-eyed Vireo) upon returning to the Texas A&M University campus, for a still-standing personal record of 25 lifers in one day, beating the previous entry on this list.
December 9, 2010: my 22nd birthday; after finishing that day's finals at HSU, I went birding at Clam Beach north of Arcata with one of my best friends and classmates, picking up an exceptional rarity (overwintering Brown Shrike) as a lifer, plus another, less spectacular life bird as well (Snowy Plover).
January 10, 2012: first time getting a "sapsucker slam" (all 4 species) at Veterans' Park in Sylmar, Los Angeles; none were lifers by then, but after getting the 2 rarer species (Red-naped and Yellow-bellied Sapsucker) shortly after arrival, I knew such a feat was possible. Ironically, what is ostensibly the most common of the four in the L.A. area (Red-breasted Sapsucker) took the longest to find, but I eventually managed to do it.
January 16, 2012: an ordinary day of birding in the Arcata Bottoms turns extraordinary when I discover a female Lark Bunting at the Lanphere Rd. dairy; in addition to being a lifer, it's the first rare bird find I could claim as my own (the first winter record of the species in northwestern California, and it stayed in the area for several months).
March 23, 2013: after driving for 10 hours through the night with some other birders from the Humboldt area, we took a crazy trip across western Washington state: picked up 4 Snowy Owls (lifers) at dawn at Ocean Shores; Slaty-backed Gull (also a lifer) at mid-day in Tacoma; and dipped on Bohemian Waxwings (a would-be lifer) in East Wenatchee at dusk.
November 1, 2014: saw California's 2nd Olive-backed Pipit (a lifer, needless to say), found earlier that day at Yorba Regional Park in Anaheim, California. I'd chased rare birds before, and I'd seen crowds looking for them, but I'd never seen a crowd like that before.
These are but a sampling of my many memorable birding days. I keep a word document with all these dates and many others that acts as a sort of "diary" of my time as a birder, for future reference should I ever decide to write a memoir.
I'll be making my first trip to southeast Arizona in the next few weeks, during which I hope and expect to reach that #500 milestone with one of the local specialties. Another memorable day awaits soon...