My intuition is that since it's a 1/1 when I declare it as an attacker, the goblin should proc the Cavalcade, but I'm having second thoughts since rule 508.1k ("Each chosen creature still controlled by the active player becomes an attacking creature," which is when the static ability of the Oriflamme should kick in) comes before 508.1m ("Any abilities that trigger on attackers being declared trigger," which is when Cavalcade would trigger).
It will not trigger. It's a weird issue of timing, but basically the way the game handles triggered abilities it checks immediately after the event to see if it should trigger. Because the creature will have the static buff as soon as it is declared an attacker it will not trigger the Cavalcade.
In this scenario, as soon as an otherwise 1/1 creature you control becomes an attacking creature, it will immediately get +1/+0 (C.R. 613.1g, 613.3c, 613.4, 611.3b, 113.6), so will become 2/1, so that Cavalcade of Calamity's ability won't trigger for that creature (C.R. 508.1k, 508.1m). (It won't trigger this way even if the game understands abilities "that trigger on attackers being declared" to trigger simultaneously on those attackers being declared [C.R. 603.10; see also C.R. 603.2].)
EDIT (Feb. 19, 2020): One rule was renumbered with Core Set 2020. Corrected another rule citation.
My intuition is that since it's a 1/1 when I declare it as an attacker, the goblin should proc the Cavalcade, but I'm having second thoughts since rule 508.1k ("Each chosen creature still controlled by the active player becomes an attacking creature," which is when the static ability of the Oriflamme should kick in) comes before 508.1m ("Any abilities that trigger on attackers being declared trigger," which is when Cavalcade would trigger).
What happens here?
EDIT (Feb. 19, 2020): One rule was renumbered with Core Set 2020. Corrected another rule citation.
Thanks for your quick answers