Contacting Mercury

I am going to create a magickal square and contact Mercury to help me interpret the tarot. Once im done with the reading, do i dismiss him or just go about my day?

Usually it’s in good taste to ask the entity to depart. If you’re okay with it hanging around and leaving its energy then by all means don’t formally end the evocation.
Try these.

Try this Step 1: Clear the Deck. Shuffle the cards while maintaining a clear mind
with no thought of the reading that is to come. This effectively clears the deck
of remaining patterns that would have emerged from the last reading it gave.
Step 2: Invoke. Hold the deck in the left hand and place the right hand
over it in a gesture of invocation. The powers that you choose to invoke are
up to you. Here is what I use:
In the name of Elohim Tzabaoth I Invoke thee HERMES.
Who knows all things between Heaven and Hades.
O Master of the magick of mercury,Lay your hand upon these cards,
Consecrate them,
Make them reveal truly the mysteries of time and possibility.
Step 3: Programming. Keeping the deck in the same position as it was in
your invocation, close your eyes and imagine that you are walking up a hill
toward a cliffs edge. The fool is at that edge appearing exactly as he does in the
deck that you are using. Walk up to him and tell him the question that you
want answered or the name of the person you are reading for. Also tell him
the type of layout you will be using—this is most important if you use multiple
layouts. Turn and walk away from the cliff, than open your eyes, shuffle until
you feel that the deck has been shuffled properly, and begin your reading. As
the zero card in the deck, the fool has the ability to transmit messages to all
the other genii of the deck, just as the winged helmet and sandals of the god
Mercury enable him to move between worlds. The three steps of the previous
method effectively invoke the spirit of mercury in the macrocosm and channel
it into the microcosm of the deck that you are using. If you read with playing
cards instead of Tarot you can use the Joker instead of the fool to do the same
programming exercise.

I’ ve done a mercury ritual using David Griffin’s book couple of weeks ago. I did dismiss the spirits by knocking on my altar and giving them a license to depart. I didn’t do any banishing afterwards and I’ve skipped the banishing part of the ritual. It worked quite well and I saw results the next day. I did slightly modify the ritual but it should work well as given in the book.