Words like まいにち, よく, and ときどき say how often something happens, with no particle attached.
JLPTN5
[frequency word] + [verb phrase]
Frequency adverbs sit before the verb phrase — usually right after the topic marked with は — and never take a particle of their own: まいにちべんきょうします (studies every day), ときどきあそびます (sometimes plays).