Today is indeed Robbie Burns birthday, and haggis is the traditional dinner, along with recitation of his Ode to a Haggis. You can look it up on google. The other poem John is muttering to himself is called To a Mouse, and it's adorable. :)
And for the curious - I share John's enthusiasm when it comes to haggis, I love the stuff!
Mrs. Hudson let herself in the front door and wrinkled her nose, wondering what on earth the detective upstairs was up to, and spared a quick thought for his poor doctor. However, for once, the pungent smell of offal and organ coming from the flat had nothing whatsoever to do with Sherlock, or any of his revolting experiments.
John puttered contentedly about in the kitchen-cum-laboratory, humming to himself as he mixed up the ground heart, liver, and lungs, and the chopped onions, carefully mixing them with cooked oats and lamb fat. The odour was overpowering, and only got more noticeable as John stared at the spice rack for a moment and carefully added in pepper and nutmeg. Sherlock could hear him quietly murmuring something about a "wee timorous beastie" and a "murdering pattle" as John meticulously tucked the disgusting mixture into an empty stomach, of all things.
"You foul, wretched descendant of a filthy Scot." Sherlock muttered, scrunching up his nose. "I hope you don't expect me to partake in the consumption of that abomination."
Smiling and rolling his eyes indulgently, John carefully lowered the dark, dense little ball into the simmering water on the stovetop. "Haggis is absolutely delicious, Sherlock. Trust me, you'll like it. Besides, it's a traditional way to celebrate the anniversary of the birth of Robert Burns."