To determine if the current day is a "Shade of the Revenant" (for making Black Soul Gems at one of the four altars, Dark Fissure being one), enter this command from the console:

show MGNecroAnchorites.Halo

If the value returned is 1.0000, then the Shade is present, 0.0000 if not.

In the script where this variable is found (has NecroAnchorites in the name, I'll check at home tonight and edit the post with the exact name), there's an if-statement where a day # is tested for < 7. I changed the 7 to a 2 using the Construction Set, and sure enough, the Shade occurred on "Days Passed == 4", instead of the Shade occurring on "Days Passed == 9" when the value is 7 as normal.

Also, if you visit one of the four sites with an altar, I think the Worm Anchorite only shows up if the "Necromancer's Moon" quest of the Mages Guild is running. So enjoy making those Black Soul Gems stress-free, fellow evil characters!

how do you tell if your on console

Go there, wait in sneak mode for 24 hours at a time so you come out in the nighttime, about 11 o' clock ish.

I would also save before hand so you won't have to waste 5 or 6 ingame days just write down what day it comes up on the calender then go back on that day

My character's not very good with math. When she wants to crank up some bsg's, she grabs her collection of gsg's, jumps on her mare and heads for Ra'Sava Camp on the Gold Road between Skingrad and Kvatch. She takes over the camp and uses it as a hunting base, figuring she'll be there a week or whatever. It's a wonderful camping trip. Oh, anyway, everynight, while she's cooking venison over the campfire and singing to her mare at Ra'Sava, she glances to her south. If the shade is happenin' over Fort Istirius, she'll see it. When she does, she scrambles down the hill and does her necromantic trick. EZ and fun! She's an elf after all - they live like forevveerr, so there is no hurry to race through life.

'Course now that she's a big girl, and minotaur lords walk Nirn with their lovely grand souls, she doesn't really need the necrodude gems filled with bandits anymore. She still loves hunting trips though.
:dance:

PS: Hey, we just learned the other day that if you whack the horror of dive rock, she sports a grand soul regardless of your level. Nice way for a very low level character to snag a normal grand soul - if you've got the stones for it. 'Course Buffy doesn't have any stones at all, so when she was a little girl, she went the bsg stuffed with a bandit route. Lol.

Do filled black soul gems always have grand souls, regardless of who you put in them?

No, it's just that at low levels, you can really only get grand souls by killing NPCs like Bandits and Conjurers, for example. I think Acadian mentioned one of the monsters that give grand souls later on, Minotaur Lords. I'm sure that if you had an empty Black Soul Gem and killed a regular Skeleton and did Soul Trap, it would end up containing only a petty soul.

What I mean is do all NPCs have grand souls, regardless of level.

Yes, all NPCs always have grand souls, regardless of level.

I look in my Journal, to elapsed Game Days.

Any multiple of 8 (+1) days = Revenant.

Well, the UESP wiki page says that's normally true, but it also says that if you spend any time in jail, then the formula can be thrown off. Hence the "true/false" method of using the console to be sure. Or like Acadian said, just camp out at night and look for the pretty lights!

Days Passed Calendar Date
1 Last Seed 27
9 Heartfire 4
17 Heartfire 12
25 Heartfire 20
33 Heartfire 28
41 Frostfall 6
49 Frostfall 14
57 Frostfall 22
65 Frostfall 30

it happens ever 8 game days hope that helps