This is the 4364th most frequent German word.
gelegentlich
Occasionally.
In this sentence, 'gelegentlich' is used as an adverb to indicate the frequency of the activity, meaning 'occasionally' or 'now and then.'
Ich lese gelegentlich ein Buch.
I occasionally read a book.
Here, 'gelegentliches' modifies the noun 'Regen' as an adjective, meaning 'occasional' or 'happening sometimes.'
Gelegentliches Regen kann die Veranstaltung beeinflussen.
Occasional rain can affect the event.
This usage of 'gelegentlich' again functions as an adverb, describing how frequently the action 'treffen' (to meet) happens.