So...

As of today, I've started my bulking regiment. I'm 20 years old, 5'11", 150 pounds (looking to put on 15-20 pounds of muscle).

I'm doing a Push/Pull/Legs workout Monday, Wednesday, and Friday. On Tuesday and Thursday I'm doing a 20-30 minute fasted HIIT session and on Tuesday, Thursday, and Saturday I'm doing abdominal work.

If I consider myself "highly active", I need ~3000 calories to maintain my body weight. So I'm eating 3500 calories a day to gain a pound per week. But I would only say that I'm "highly active" on my lifting days. The other days would probably be closer to "moderately active" (since I'm only doing HIIT and abs).

Here is my question: Should I only consume 3500 calories on my lifting days and decrease them on my rest days? Or should I just eat 3500 calories per day whether I'm lifting that day or not?

Hopefully that question makes sense. Thanks for the help.

If I were you, I would keep the calories up each day. Some drop their calories on non-training days. However, I started with a similar build as you. I was 5'11 and 145-150ish when I first started training. I was really only "very active" on training days. I first tried 3500 calories on training days only, and at maintenance on non-training days. I did not gain an ounce. I then bumped it up to 3500 cals every day, and still did not gain an ounce. Finally, I bumped it up 250 calories every 2 weeks until I reached 4000ish cals pretty much everyday and I have been gaining since.

Each of us are metabolically different and these calculators are good for giving you an idea of where to start, but that is about it. Your best bet is to start with 3500 either every day or only on training days and track your numbers. Take your body fat readings, weigh yourself, take measurements. If nothing is changing, or things are not going the way you want them, make the change!

Far too many individuals say, "I am going to bulk." They calculate their calories required using primitive tools, and start eating that many calories from there on out hoping that their "bulking program" is working. That does not cut it. You must be proactive with bulking. Consistently bump your calories up until you start getting the desired results.

Good luck.

You need consistency. best thing to do, is go with your 3,500 calories a day estimate, for 2 weeks. make sure you weigh in and get body fat tested at the start of the bulk, and at the end of 2 weeks.

if you don't even gain 1lb, you probably need a few more calories. If you gain 3lbs of fat, you're eating too much.
you need the bodyfat test so you know if the weight you're gaining is lean mass or fat. it IS possible to bulk with minimal fat gain. you don't have to gain a lb of fat for every lb of muscle

Word to both.

Keep them up, and see how it goes.