What are the crosses on the wall almanac for the Heathcliff keeps?

Heathcliff uses the crosses to mark each and every time that Catherine visits the Lintons.

'Nothing - only look at the almanack on that wall;' he pointed to a framed sheet hanging near the window, and continued, 'The crosses are for the evenings you have spent with the Lintons, the dots for those spent with me. Do you see? I've marked every day.'