Monica . . . . . . . . since last week.

'Did plan' (more specifically 'planned') is the simple past form of the verb 'plan'.
Simple past tense is used to indicate an action which took place in the past time and has no effect in the future.
''Has been planning" is the present perfect continuous form of the verb 'plan'.
Present perfect continuous tense is used to indicate an action which took place in the past yet has its effect at present.
''Had been planning" is the past perfect continuous tense of the verb 'plan'.
Past perfect continuous tense is used to indicate an action which took place at some time in the past and had its effect till sometime in the past.
"Was planning" is the past continuous tense of the verb 'plan'.
Past continuous tense is used to indicate an action which had been going on in the past.